p i 
 
 J 7 
 
 ' 
 
COLLOID CHEMISTRY 
 
 AN INTRODUCTION, WITH SOME 
 PRACTICAL APPLICATIONS 
 
 BY 
 JEROME ALEXANDER, M.Sc., 
 
 Chairman, Special Committee on Colloids, Division of Chemistry 
 
 and Chemical Technology, National Research Council 
 
 Member Amer. Institute Chemical Engineers 
 
 ILLUSTRATED 
 
 NEW YORK 
 
 D. VAN NOSTRAND COMPANY 
 
 25 PARK PLACE 
 
 1919 
 
J>> 
 
 COPYRIGHT, .1919, 
 
 BY 
 D, VAN NOSTRAND COMPANY 
 
 Stanbope 
 
 F. H. GILSON COMPANY 
 BOSTON, U.S.A. 
 
PREFACE 
 
 THIS little book is the result of an attempt 
 to compress within a very limited space, the 
 most important general properties of colloids, 
 and some of the practical applications of col- 
 loid chemistry. Its object will be accomplished 
 if it is helpful in extending the sphere of in- 
 terest in this fascinating twilight zone between 
 physics and chemistry. 
 
 J. A. 
 
 NEW YORK, 
 Nov. 1, 1918. 
 
 415477 
 
TABLE OF CONTENTS 
 
 CHAPTEB PAGE 
 
 I. INTRODUCTION 1 
 
 II. CLASSIFICATION OF COLLOIDS 10 
 
 III. CONSEQUENCES OF SUBDIVISION 14 
 
 IV. THE ULTRAMICBOSCOPE 17 
 
 V. GENERAL PROPERTIES OF COLLOIDS 24 
 
 VI. PRACTICAL APPLICATIONS OF COLLOID CHEMISTRY . 36 
 
 BIBLIOGRAPHY 85 
 
 AUTHOR INDEX 87 
 
 SUBJECT INDEX 89 
 
COLLOID CHEMISTEY 
 
 CHAPTER I 
 Introduction 
 
 Although many facts and principles con- 
 cerning colloids have from time immemorial 
 been known and utilized empirically, the 
 scientific foundation of modern colloid chemis- 
 try was laid by an Englishman, Thomas Gra- 
 ham, F.R.S., Master of the Mint. In two 
 basic papers on this subject, the first entitled 
 "Liquid Diffusion Applied to Analysis," read 
 before the Royal Society of London, June 13, 
 1861, the second entitled "On the Properties 
 of Colloidal Silicic Acid and other Analogous 
 Colloidal Substances," published in the Pro- 
 ceedings of the Royal Society, June 16, 1864, 
 Graham pointed out the essential facts regard- 
 ing colloids and the colloidal condition, and 
 established much of the nomenclature in use 
 
 at the present day. In the first of these 
 
 i 
 
2 \ COLLOID CHEMISTRY 
 
 papers Graham says: "The property of vola- 
 tility, possessed in various degrees by so many 
 substances, affords invaluable means of separa- 
 tion, as is seen in the ever-recurring processes 
 of evaporation and distillation. So similar in 
 character to volatility is the diffusive power 
 possessed by all liquid substances, that we 
 may fairly reckon upon a class of analogous 
 analytical resources to arise from it. The 
 range also in the degree of diffusive mobility 
 exhibited by different substances appears to be 
 as wide as the scale of vapor tensions. Thus 
 hydrate of potash may be said to possess double 
 the velocity of diffusion of sulphate of potash, 
 and sulphate jti potash again double the 
 velocity of sugar, alcohol and sulphate of 
 magnesia. But the substances named belong 
 all, as regards diffusion, to the more "vola- 
 tile " class. The comparatively "fixed " class, 
 as regards diffusion, is represented by a differ- 
 ent order of chemical substances, marked out 
 by the absence of the power to crystallize, 
 which are slow in the extreme. Among the 
 latter are hydrated silicic acid, hydrated alu- 
 mina and other metallic peroxids of the 
 aluminous class, when they exist in the soluble 
 
INTRODUCTION 3 
 
 form; with starch, dextrin and the gums, 
 caramel, tannin, albumen, gelatin, vegetable 
 and animal extractive matters. Low diffusi- 
 bility is noj^the only property which the bodies 
 last enumerated possess in common. They are 
 distinguished by the gelatinous character of 
 their hydrates. Although often largely soluble 
 in water, they are held in solution by a most 
 feeble force. They appear singularly inert in 
 the capacity of acids and bases, and in all the 
 ordinary chemical relations. But, on the other 
 hand, their peculiar physical aggregation with 
 the chemical indifference referred to appears 
 to be required in substances that can intervene 
 in the organic processes of life. The plastic 
 elements of the animal body are found in this 
 class. As gelatin appears to be its type, it is 
 proposed to designate -substances of this class 
 as colloids, and to speak of their peculiar form 
 of aggregation as the colloidal condition of 
 matter. Opposed to the colloidal is the crys- 
 talline condition. Substances affecting the 
 latter form will be classed as crystalloids. The 
 distinction is no doubt one of intimate molec- 
 ular constitution. 
 
 " Although chemically inert in the ordinary 
 
4 COLLOID CHEMISTRY 
 
 sense, colloids possess a compensating activity 
 of their own, arising out of their physical 
 properties. While the rigidity of the crystal- 
 line structure shuts out external impressions, 
 the softness of the gelatinous colloid partakes 
 of fluidity, and enables the colloid to become 
 a medium for liquid diffusion, like water itself. 
 The same penetrability appears to take the 
 form of cementation in such colloids as can 
 exist at high temperature. Hence a wide 
 sensibility on the part of colloids to external 
 agents. Another and eminently character- 
 istic quality of colloids is their mutability. 
 Their existence is a continued metastasis. A 
 colloid may be compared in this respect to 
 water, while existing liquid at a temperature 
 under its usual freezing-point, or to a super- 
 saturated saline solution. Fluid colloids ap- 
 pear to have always a pectous modification; 
 and they often pass under the slightest 
 influences from the first to the second condi- 
 tion. The solution of hydrated silicic acid, 
 for instance, is easily obtained in a state of 
 purity, but it cannot be preserved. It may 
 remain fluid for days or weeks in a sealed tube, 
 but is sure to gelatinize and become insoluble 
 
INTRODUCTION 5 
 
 at last. Nor does the change of this colloid 
 appear to stop at that point. For the mineral 
 forms of silicic acid deposited from water, such 
 as flint, are often found to have passed, during 
 the geological ages of their existence, from the 
 vitreous or colloidal into the crystalline con- 
 dition. (H. Rose.) The colloidal is, in fact, 
 a dynamical state of matter, the crystalloidal 
 being the statical condition. The colloid 
 possesses Energia. It may be looked upon 
 as the probable primary source of the force 
 appearing in the phenomena of vitality. To 
 the gradual manner in which colloidal changes 
 take place (for they always demand time as an 
 element) may the characteristic protraction of 
 chemico-organic changes also be referred. . . . 
 
 "It may perhaps be allowed to me to apply 
 the convenient term dialysis to the method of 
 separation by diffusion through a septum of 
 gelatinous matter. The most suitable of all 
 substances for the dialytic septum appears to 
 be the commercial material known as vegetable 
 parchment, or parchment-paper. . . ." 
 
 At the beginning of the second paper above 
 referred to, Graham states: "The prevalent 
 notions respecting solubility have been de- 
 
6 COLLOID CHEMISTRY 
 
 rived chiefly from observations on crystalline 
 salts, and are very imperfectly applicable to 
 the class of colloidal solutions." From this 
 it may be seen that Graham appreciated the 
 fact that all the laws of crystalloidal solutions 
 could not be applied to colloidal solutions. 
 In the case of crystalloidal solutions the dis- 
 solved substance is present in a state of molec- 
 ular subdivision, and, according to the ioniza- 
 tion theory, is in many cases dissociated into 
 ions. With colloidal solutions, on the other 
 hand, we have a lesser degree of subdivision, 
 and the particles in solution are larger and 
 more cumbersome. As Graham remarked, 
 "The inquiry suggests itself whether the 
 colloid molecule may not be constituted by the 
 grouping together of a number of smaller 
 crystalloid molecules, and whether the basis 
 of colloidality may not really be this composite 
 character of the molecule." This is to-day 
 the idea generally accepted. 
 
 COLLOID CHEMISTRY DEFINED 
 
 Colloid chemistry deals with the behavior 
 and properties of matter in the colloidal con- 
 dition, which, as we now know, means a certain 
 
INTRODUCTION 7 
 
 very fine state of subdivision. While there 
 are no sharp limitations to the size particles in 
 colloidal solutions, it may in a general way be 
 stated that their sphere begins with dimensions 
 somewhat smaller than a wave length of light, 
 and extends downward well into dimensions 
 which theory ascribes to the molecules of 
 crystalloids. (See Table II, p. 12.) 
 
 SUSPENSION vs. SOLUTION 
 
 With the aid of the ultramicroscope, which 
 renders visible particles approaching in mi- 
 nuteness molecular dimensions, Zsigmondy has 
 shown that there is no sharp line of demarca- 
 tion between suspensions and solutions, but 
 that with increasing fineness in the subdivision 
 of the dissolved substance, there is a progres- 
 sive change in the properties of the resulting 
 fluids, the influence of gravity gradually 
 yielding to that of the electric charge of 
 particles, of surface tension and of other forms 
 of energy. Thus in the case of metallic gold, 
 subdivisions whose particles are 1 M and over 
 act as real suspensions and deposit their gold, 
 whereas much finer subdivisions (60 AM and 
 under) exhibit all the properties of metal 
 
8 COLLOID CHEMISTRY 
 
 hydrosols or colloidal solutions. In the ultra- 
 microscope the coarser subdivisions show the 
 well-known Brownian movement, which 
 greatly increases as the particles become- 
 smaller, until at the present limit of ultra- 
 microscopic visibility (about 5 MM) it becomes 
 enormous both in speed and amplitude. 
 
 On the other hand, there is no sharp dis- 
 tinction between colloids and crystalloids, but 
 as the particles in solution become smaller and 
 smaller, the optical heterogeneity decreases 
 correspondingly, finally vanishing as molec- 
 ular dimensions are approached.* That even 
 crystalloid solutions are not in a strict sense 
 homogeneous, is indicated by an experiment 
 of van Calcar and Lobry de Bruyn (Rec. Trav. 
 
 * In an article entitled "Pedetic Motion in Relation to 
 Colloidal Solutions " published in Chemical News, 1892, Vol. 
 65, p. 90, William Ramsay, Ph.D., F.R.S, (afterward Sir 
 William Ramsay), clearly expressed this view in the fol- 
 lowing words: "I am disposed to conclude that solution 
 is nothing but subdivision and admixture, owing to attrac- 
 tions between solvent and dissolved substance accompanied 
 by pedetic motion; that the true osmotic pressure has, 
 probably, never been measured; and that a continuous pas- 
 sage can be traced between visible particles in suspension 
 and matter in solution; that, in the words of the old adage, 
 Natura nihil Jti per sattum." 
 
INTRODUCTION 9 
 
 chim. Pays-Bas, 1904, 23, 218), who caused 
 the crystallization of a considerable part of 
 saturated crystalloid solutions at the periph- 
 ery of a rapidly rotating centrifuge. 
 
CHAPTER II 
 Classification of Colloids 
 
 The broadest classification of colloids is that 
 of Wolfgang Ostwald (Koll. Zeitschr., Vol. 1, 
 page 291), who grouped them according to the 
 physical state (gaseous, liquid or solid) of the 
 subdivided substance (dispersed phase), and 
 of the medium in which the particles of the 
 subdivided substance are distributed (disper- 
 sion medium).* Table I (page 11) shows the 
 nine resulting groups and gives some instances 
 of each. 
 
 Ostwald's classification, however, is more 
 theoretical than practical, for the properties of 
 colloids are dependent mainly upon the specific 
 nature of the dispersed substance and its degree 
 of subdivision. Following Hardy, Zsigmondy 
 divided colloids into two classes, the reversible 
 and irreversible; the former redissolve after 
 
 * G. Bredig proposed to call colloids "microheterogeneous 
 systems." W. Ostwald called them "dispersed heterogeneous 
 systems," which expression was contracted by P. P. von 
 Weimarn into the term "dispersoids." 
 
 10 
 
CLASSIFICATION OF COLLOIDS 
 
 11 
 
 desiccation at ordinary temperatures, whereas 
 the latter do not. 
 
 TABLE I 
 
 Dispersed 
 phase. 
 
 Dispersion 
 medium. 
 
 Example. 
 
 Gas 
 
 Gas 
 
 No example, since gases are miscible in all 
 
 
 
 proportions. 
 
 Gas 
 
 Liquid 
 
 Fine foam, gas in beer. 
 
 Gas 
 
 Solid 
 
 Gaseous inclusions in minerals (meer- 
 
 
 
 schaum, pumice), hydrogen in iron, oxy- 
 
 
 
 gen in silver. 
 
 Liquid 
 
 Gas 
 
 Atmospheric fog, clouds, gases at critical 
 
 
 
 state. 
 
 Liauid 
 
 Liquid 
 
 Emulsions of oil in water, cream, colloidal 
 
 .LJ114 UJ.U. . ...... 
 
 
 water in chloroform. 
 
 Liquid 
 
 Solid 
 
 Mercury in ointments, water in paraffin wax, 
 
 
 
 liquid inclusions in minerals. 
 
 Solid 
 
 Gas 
 
 Cosmic dust, smoke, condensing vapors, 
 
 
 
 (ammonium chlorid). 
 
 Solid 
 
 Liquid 
 
 Colloidal gold, colloidal sodium chlorid, col- 
 
 
 
 loidal ice in chloroform. 
 
 Solid 
 
 Solid 
 
 Solid solutions, colloidal gold in ruby glass, 
 
 
 
 coloring matter in gems. 
 
 Table II, taken from Zsigmondy,* illustrates 
 this classification, and shows how colloids hav- 
 ing the same particle size or degree of sub- 
 division may nevertheless act quite differently 
 because of specific differences in the nature of 
 the dispersed substances. 
 
 * Colloids and the Ultramicroscope, J. Wiley & Son, Inc. 
 (Translation by J. Alexander.) 
 
12 COLLOID CHEMISTRY 
 
 With the reversible colloids (gelatin, gum 
 arabic, albumen), there is a more intimate 
 union between the two phases; in fact it is 
 probable that with them we have really a 
 mixture of (1) a dispersed phase of water sub- 
 divided in the solid, with (2) a dispersing 
 phase of the solid finely subdivided in water. 
 The former are therefore called emulsoids and 
 the latter suspensoids. Colloids of the rever- 
 sible type are also said to be hydrophile or 
 lyophile, while the irreversible colloids are 
 hydrophobe or lyophobe. 
 
 No sharp line is to be drawn, however, for 
 besides intermediate or transition cases be- 
 tween the two classes, there may be recognized 
 two groups of irreversible colloids, roughly 
 defined by their behavior upon concentration: 
 
 First: The completely irreversible , which 
 coagulate while still quite dilute and separate 
 sharply from the solvent with the formation of 
 a pulverulent precipitate rather than a gel 
 (i.e., pure colloidal metals). Chemical or 
 electrical energy is needed to bring them back 
 again into colloidal solution. 
 
 Second: The incompletely reversible which, 
 when quite concentrated, form a gel that may 
 
CLASSIFICATION OF COLLOIDS 13 
 
 be easily redissolved or peptisized by com- 
 paratively small amounts of reagents, unless 
 the evaporation has proceeded too far (i.e., 
 colloidal stannic acid). 
 
CHAPTER III 
 Consequences of Subdivision 
 
 As the subdivision of a substance proceeds, 
 the area of its effective surface increases enor- 
 mously, as maybe seen from the following Table 
 III adapted from Ostwald. Consequently sur- 
 face forces, such as adsorption, capillarity and 
 surface tension, become enormously magnified 
 and of primary importance. Furthermore, the 
 so-called radius of molecular attraction (p = 
 50 MM) is well within the colloidal field, so that 
 the specific attractive forces of the particles 
 also enter as a controlling factor. In fact, 
 before substances can unite chemically their 
 particles must be first brought into proper 
 subdivision and proximity,* by solution, fusion, 
 ionization or even by mere pressure, as was 
 demonstrated by W. Spring, who caused fine 
 
 * It is a striking fact that absolutely dry sodium is not 
 attacked by absolutely dry chlorin. M. Raffo and A. Pieroni 
 observed that colloidal- suphur reduced silver salts energeti- 
 cally, whereas even fine precipitated sulphur did not form 
 silver sulphid in the cold, and did so only partially upon boiling. 
 
 14 
 
CONSEQUENCES OF SUBDIVISION 
 
 15 
 
 d .* 
 
 .3 g .. C 
 
 S * 8 1 S % s - 8 
 
 2 8 - - -' = - 
 
 II II II 
 
 <M C4 
 II II H 
 
 a a a 
 
 a s s a 
 
 g* i s 1 I 
 
 3 1 
 
 .a .a .s .s 1 1 ; 
 
 * a * 
 
 ddddddddd 
 
 * 
 
 -* o 
 
 
 
16 COLLOID CHEMISTRY 
 
 dry powders to combine chemically by high 
 pressure. If the degree of subdivision is not 
 profound enough to permit of the combination 
 of isolated atoms or ions with each other, 
 chemical combination in the strict sense may 
 not occur, but there may be produced " ad- 
 sorption compounds " resulting from the union 
 of atomic or ionic mobs in indefinite or non- 
 stoichiometric proportions, under the influence 
 of more or less modified chemical forces. The 
 combination of arsenious acid and ferric oxid 
 which Bunsen regarded as a basic ferric arsen- 
 ite, 4 Fe 2 0s, A^Oa, 5 H 2 0, has been shown by 
 Biltz and Behre to be an adsorption compound; 
 and Zsigmondy proved "Purple of Cassius" to be 
 an adsorption compound of colloidal gold and 
 colloidal stannic acid by actually synthesizing it 
 by mixing the two separate colloidal solutions. 
 The effect of increasing subdivision upon the 
 particles in colloidal solutions is illustrated in 
 Table IV, adapted from Zsigmondy. Tables 
 V and VI were prepared by Zsigmondy to 
 illustrate visually the relation of the sizes of 
 colloidal particles to well-known microscopic 
 objects on the one hand and to the theoretical 
 sizes of molecules on the other. 
 
TABLE V 
 LINEAR MAGNIFICATION 1 : 10,000 
 
 A. Human blood corpuscles (diameter 7.5 n, thickness 1.6 p). 
 
 B. Fragment of rice starch granule (according to v. Hohnel) 3-8 /*. 
 
 C. Particles in a kaolin suspension. 
 
 E. Anthrax bacillus (length 4-15 n, width about 1 /*). 
 
 F. Cocci (diameter about 0.5-1 n, rarely 2 /*) 
 
 f, g, h. Particles of colloidal gold solutions Au 73a , Au^, Au ( 0.006-0.015 ft), 
 i, k, 1. Particles from settled gold suspensions (0.075-0.2 n). 
 
TABLE VI 
 LINEAR MAGNIFICATION 1 : 1,000,000 
 
 a b 
 
 ? D m S 
 
 e ei e 2 f g 
 
 n 
 
 a-d. Hypothetical Molecular Dimensions 
 
 a. Hydrogen molecule dia. 0.1 nil. 
 
 b. Alcohol molecule dia. 0.5 MM- 
 
 c. Chloroform molecule dia. 0.8 pp. 
 
 d. Molecule of soluble starch dia. about 5 MM 
 
 e-h. Gold Particles in Colloidal Gold Solutions 
 
 e. Gold particle in Aui (too small to determine), 
 d. " " " " , about 1.7 MM- 
 
 ej. " " " " , " 3.0 MM- 
 /. " " Au 73a , 6 MM- 
 
 g. " " " Au92, " 10 MM- 
 h. " " " AUOT, " 15 MM- 
 u Gold particle in settled gold suspension. 
 
CONSEQUENCES OF SUBDIVISION 17 
 
 s 
 
 .2 
 
 1 
 
 a. 
 
 
 3 
 
 c3 
 
 I 1 11 
 
 II I 1 If 
 
 l! 11 I 
 
 & 
 
 3 3 ^3 03 
 
 fi( O* o f^ 
 
 ! 1 
 
 II i i 
 os I I 
 
 a 1 
 
 '111 E S " " 
 
 P 
 
CHAPTER IV 
 The Ultramicroscope 
 
 As this instrument revolutionized colloid 
 research, a brief description of it is essential. 
 
 It is a matter of every-day experience that the 
 unseen motes and dust particles in the air be- 
 come visible in a beam of bright light, espe- 
 cially against a dark ground, and in this simple 
 fact lies the principle of the ultramicroscope. 
 
 Faraday and later Tyndall made use of a 
 convergent beam of light to demonstrate the 
 optical inhomogeneity of solutions; for in 
 fluids not optically clear, the path of the beam 
 becomes more or less distinctly visible, because 
 of the light scattered by the particles present. 
 In this manner can be recognized much smaller 
 quantities of matter than by spectrum analysis 
 in fact less than 10~ 8 mg. (1/10,000,000) of 
 metallic gold can thus be detected with the 
 naked eye. 
 
 Prof. Richard Zsigmondy while experiment- 
 ing with colloidal solutions conceived the idea 
 of examining this light cone microscopically. 
 
 18 
 
THE ULTRAMICROSCOPE 19 
 
 His preliminary experiments having demon- 
 strated that he could thus see the individual 
 particles in various hydrosols, he sought the 
 assistance of Dr. H. Siedentopf, scientific 
 director of the Carl Zeiss factory, in Jena, 
 where was produced the first efficient ultra- 
 microscope. 
 
 The ultramicroscope consists essentially of 
 a compound microscope arranged for examin- 
 ing in a dark field an intense convergent beam 
 of light cast within or upon the substance 
 under examination. The light seen by the 
 eye represents, therefore, the light diffracted, 
 scattered or reflected upward by the substance 
 or by particles within it. 
 
 If within a thin beam of light from a pro- 
 jection lantern we scatter successively powders 
 of different substances in various degrees of 
 fineness (mica ground to pass 60, 100 and 160 
 mesh; lampblack; powdered oxid of zinc; 
 flake and powdered graphite), some of them 
 will produce only a homogeneous illumination 
 of the beam in which no isolated particles can 
 be seen, whereas with others, the individual 
 particles are distinctly visible. 
 
 Passing the beam through a beaker of dis- 
 
20 COLLOID CHEMISTRY 
 
 tilled water, nothing can be seen; but upon 
 the addition of a faw drops of colloidal gold 
 solution, which appears quite clear to trans- 
 mitted light, the path of the beam through the 
 fluid immediately becomes visible. This Tyn- 
 dall effect,* as it is called, might be considered 
 a criterion of colloidal solution were it not that 
 very minute traces of colloidal impurities can 
 produce it and it is often exhibited by solu- 
 tions generally regarded as crystalloidal 
 those of many dyestuffs for example; further- 
 more with increasing fineness of subdivision 
 the Tyndall effect decreases, disappearing as 
 molecular dimensions are approached. 
 
 Just as in the cosmic field our most powerful 
 telescopes fail to resolve the fixed stars, which 
 are nevertheless visible as points of light of 
 varying brilliancy, so, too, in the ultramicro- 
 scopic field, we can see particles much smaller 
 than the resolving power of the microscope 
 (that is, smaller than a wave length of light) 
 provided only that they diffract sufficient light 
 to affect the retina. Based upon the experi- 
 ence of astronomers we may be able greatly to 
 increase the sensitiveness of the ultramicro- 
 
 * Also known as the Faraday-Tyndall effect. 
 
THE ULTRAMICROSCOPE 21 
 
 scope by fortifying the eye, so to speak, with 
 the photographic plate, using at the same time 
 tropical sunlight or ultraviolet light for illu- 
 mination. 
 
 In the original form of the ultramicroscope, 
 as perfected by Siedentopf and Zsigmondy, 
 which is the one best adapted for the examina- 
 tion of transparent solids, a side illumination 
 is effected by a microscope objective with 
 micrometer movements, which throws an 
 intense but minute conical beam of light into 
 the fluid contained in a little cell having 
 quartz windows at the side and top. Above 
 this cell a compound microscope is adjusted 
 vertically, so that the narrowest part of the 
 light cone occupies the center of the focal 
 plane. If the fluid under examination is op- 
 tically clear or if it contains particles so small 
 that they cannot diffract sufficient light to 
 create a visual impression, the light cone 
 cannot be seen. If enough light is diffracted, 
 the light cone becomes visible, being homo- 
 geneous if the particles are too small or too 
 close together to be individually seen, and 
 heterogeneous if the particles can be individu- 
 ally distinguished. Particles or dimensions 
 
22 COLLOID CHEMISTRY 
 
 beyond the resolving power of the microscope 
 (about J At) are for brevity termed ultrami- 
 crons. Ultramicrons that can individually be 
 made visible are called submicrons (or hypo- 
 microns) while those so small that they 
 produce an unresolvable light cone are termed 
 amicrons. 
 
 Knowing the percentage of gold present in 
 a colloidal gold solution and assuming a certain 
 specific gravity and uniform shape for the gold 
 particles, the average size and mass of a single 
 particle of colloidal gold can be calculated, if 
 the number present in a given volume be first 
 counted. In this manner Zsigmondy has 
 shown that the smallest particles of colloidal 
 gold which can be individually distinguished 
 with bright sunlight, are approximately 5 /*/* 
 in diameter, that is, five-millionths of a milli- 
 meter; still smaller particles exist but they 
 produce only an unresolvable light cone. 
 Magnified 1,000,000 times such a tiny gold 
 particle would be about J inch in diameter, 
 while a human red blood corpuscle would be 
 about 25 feet across, and a hydrogen molecule 
 a speck barely visible. The gold particles in 
 the unresolvable light cone must therefore 
 
THE ULTRAMICROSCOPE 23 
 
 closely approach molecular dimensions. In 
 fact, by allowing amicrons to grow into visi- 
 bility in a suitable solution and then counting 
 them, Zsigmondy has recently shown that 
 some of the particles of colloidal gold have 
 a mass of 1-5. 10~ 16 mg., indicating a size of 1.7 
 to 3 jjifji. 
 
 Various other types of ultramicroscopes, 
 mainly modifications of dark field illumination, 
 have been developed by Cotton and Mouton, 
 Ignatowski (made by Leitz), Siedentopf (car- 
 dioid condenser, made by Zeiss) and others, 
 and besides being useful in examining colloidal 
 solutions, they have enabled pathologists to 
 see and discover ultramicroscopic bacteria 
 (spirochetes, infantile paralysis). 
 
 Bausch & Lamb Optical Co. of Rochester, 
 N. Y., are now producing a useful ultramicro- 
 scope. 
 
CHAPTER V! 
 General Properties of Colloids 
 
 The optical properties of colloids and their 
 simulation of chemical compounds have been 
 already referred to. The other general proper- 
 ties of colloids may be considered under the 
 following headings: 
 
 1. Colloidal Protection. 
 
 2. Dialysis, Ultrafiltration and Diffusion. 
 
 3. Electric Charge and Migration. 
 
 4. Pectization (Coagulation) and Peptiza- 
 
 tion. 
 
 COLLOIDAL PKOTECTION. A most inter- 
 esting and important fact regarding reversible 
 colloids is that they can communicate their re- 
 versible property to irreversible colloids. The 
 addition of gelatin (as little as 0.0001 per cent) 
 to a solution of colloidal gold protects the latter 
 against coagulation upon the addition of 
 electrolytes, and permits it to redissolve after 
 desiccation. Ultramicroscopic examination 
 has shown that the gelatin does not affect the 
 
 24 
 
GENERAL PROPERTIES OF COLLOIDS 25 
 
 motility of the gold particles, thus disposing of 
 the suggestion of Lobry de Bruyn that it acts 
 by decreasing their motion. The idea ad- 
 vanced by Miiller (Ber., 1904, 37, 11) that 
 gelatin acts by increasing the viscosity and 
 thus preventing the deposition of suspended 
 particles is disproved by the fact that quince 
 kernel gum, notwithstanding its viscosity, 
 exercises no protective action,* whereas the 
 small quantities of gelatin necessary to pro- 
 duce this effect cannot appreciably increase 
 the viscosity, and actually permit gold par- 
 ticles to settle if they are large enough to do so. 
 
 The action of reversible colloids in opposing 
 group formation, is of great technical impor- 
 tance, for in many cases it permits them to 
 hinder, modify and even prevent coagulation, 
 precipitation and crystallization. 
 
 DIALYSIS. Colloid solutions possess a small 
 but definite diffusibility through colloidal septa 
 (parchment paper, bladder) as was recognized 
 by Graham, who found that "tannic acid 
 passes through parchment-paper about 200 
 times slower than sodium chlorid; gum arabic 
 
 * According to Zsigmondy, quince kernel gum acts as a 
 protector with some substances. 
 
26 COLLOID CHEMISTRY 
 
 400 times slower/' Graham's original form of 
 dialyzer may be made from a wide-mouthed 
 bottle whose bottom has been removed.* The 
 mouth is closed by a piece of bladder or parch- 
 ment paper tightly bound on, the solution to 
 be dialyzed is poured in, and the bottle im- 
 mersed about halfway in water contained in a 
 larger vessel. Most of the crystalloids diffuse 
 through the membrane into the outer water, 
 which should be frequently renewed, while 
 most of the colloids remain in the original 
 bottle, and may be thus obtained in a purified 
 condition. Improved modern dialyzers con- 
 sist of parchment or collodion sacs or thimbles, 
 or even of whole bladders, which have the 
 advantage of a larger dialyzing surface. 
 
 ULTRAFILTRATION. H. Bechhold found that 
 he could make filtering membranes of varying 
 degrees of permeability by forming them from 
 jellies of varying concentration. He used prin- 
 cipally collodion dissolved in glacial acetic acid 
 and afterward immersed in water, and gelatin 
 jellies hardened in ice-cold formaldehyde. The 
 jellies were formed and hardened on pieces of 
 filter paper, which were supported from below 
 
 * A lamp chimney will answer very well. 
 
GENERAL PROPERTIES OF COLLOIDS 27 
 
 by nickel wire cloth, and clamped between 
 two flanges. The liquid to be subjected to 
 ultrafiltration is introduced in the chamber 
 thus formed and forced through the prepared 
 septum by appropriate pressure, which may 
 run up to 20 atmospheres or more and may 
 be produced by a pump or by compressed gas 
 (air, nitrogen or CO 2 ). Table VII (p. 28), pre- 
 pared by Bechhold, shows various colloids ar- 
 ranged in order of the diminishing size of 
 their particles in solution, and was obtained 
 by using ultrafilters of varying degrees of 
 porosity or permeability. 
 
 By means of ultrafiltration through ultra- 
 filters of appropriate permeability, not only 
 may colloids be separated from crystalloids, 
 but colloids having particles of different sizes 
 may be separated from each other. 
 
 DIFFUSION. Diffusion through a septum 
 is, of course, involved in dialysis. If, however, 
 diffusion occurs into a jelly, many interest- 
 ing phenomena may develop, especially if the 
 jelly adsorbs arty of the diffusing substances 
 or contains substances which can react with 
 them. 
 
 Owing to the enormous surface they present, 
 
28 COLLOID CHEMISTRY 
 
 TABLE VII 
 
 Suspensions. 
 
 Prussian blue. 
 
 Platinum sol (made by Bredig's method). 
 
 Ferric oxid hydrosol. 
 
 Casein (in milk). 
 
 Arsenic sulphid hydrosol. 
 
 Colloidal gold hydrosol (Zsigmondy's No. 4, particles 
 
 about 40 MM). 
 
 Colloidal bismuth oxid (Paal's "Bismon"). 
 Colloidal silver (Paal's "Lysargin"). 
 Colloidal silver (von Heyden's "Collargol," particles about 
 
 20 MM). 
 Colloidal gold hydrosol (Zsigmondy's No. 0, particles 
 
 about 1-4 MM). 
 Gelatin solution, 1 per cent. 
 Hemoglobin solution, 1 per cent (molecular weight about 
 
 16,000). 
 
 Serum albumin (molecular weight about 5000 to 15,000). 
 Diphtheria toxin. 
 Protalbumoses. 
 Colloidal silicic acid. 
 Lysalbinic acid. 
 Deuteroalbumoses A. 
 
 Deuteroalbumoses B (molecular weight about 2400). 
 Deuteroalbumoses C. 
 Litmus. 
 
 Dextrin (molecular weight about 965). 
 Crystalloids. 
 
 colloidal gels exhibit a powerful adsorptive 
 action. In fact, even when percolated through 
 such a relatively coarse-grained septum as sand, 
 most solutions issue with a materially reduced 
 
GENERAL PROPERTIES OF COLLOIDS 29 
 
 content of solute, and benzopurpurin solutions 
 may be thus decolorized. Further, if a solute 
 hydrolyzes into ions having different degrees 
 of adsorbability or different rates of diffusibil- 
 ity, they may be actually separated by diffusion 
 through a colloidal gel. 
 
 This phenomenon is nicely exhibited by 
 what may be termed a " patriotic test tube," 
 prepared by filling a tube about two-thirds full 
 with a slightly alkaline solution of agar contain- 
 ing a little potassium ferrocyanid and enough 
 phenolphthalsin to turn it pink. After the agar 
 has set to a firm gel, a solution of ferric chlorid 
 is carefully poured on top, and almost instantly 
 the separation becomes evident. The iron 
 forms with the ferrocyanid a slowly advancing 
 band of blue, before which the more rapidly 
 diffusing hydrochloric acid spreads a white 
 band as it discharges the pink of the indicator. 
 After the lapse of a few days the tube is about 
 equally banded in red, white, and blue. 
 
 Even then the tubes do not cease to be of 
 interest, for if they are allowed to stand several 
 weeks the pink color is all discharged and there 
 develop peculiar bands or striations of blue, 
 apparently due to the fact that the iron ferro- 
 
30 COLLOID CHEMISTRY 
 
 cyanid temporarily blocks the diffusion pas- 
 sage, which are gradually opened again after 
 a layer of the blue salt has diffused on from the 
 lower surface. 
 
 f Not only may ions be thus separated, but if 
 two solutes in the same solvent possess differ- 
 ent rates of diffusion or different degrees of 
 adsorbability, they also may be separated from 
 each other by diffusion through a colloidal gel 
 or septum. (Differential Diffusion.) 
 
 ELECTRIC CHARGE AND MIGRATION. The 
 particles of practically all colloidal solutions 
 possess an electric charge, and under the 
 influence of an electric current (difference of 
 potential) move toward the electrode having 
 the opposite charge. (Electrophoresis.) In 
 general, when two substances are brought into 
 contact, the one having the higher dielectric 
 constant becomes positively charged, whereas 
 the one with the lower dielectric constant 
 becomes negatively charged (Cohen's Law). 
 Since water has a high dielectric constant (80), 
 most substances suspended in pure water 
 become negatively charged and wander to the 
 anode. On the other hand if suspended in oil 
 of turpentine, which has a low dielectric 
 
GENERAL PROPERTIES OF COLLOIDS 31 
 
 constant (2.23), they become positively charged 
 and wander to the cathode. 
 
 If, however, electrolytes are present, Coehn's 
 law is superseded by other controlling factors, 
 such as the adsorption of ions, which may give 
 their charge to the suspended particles. In 
 fact Hardy found that in pure water albumen 
 was amphoteric; in the presence of a trace of 
 alkali it acquired a negative charge and 
 migrated to the anode; but a trace of acid gave 
 it a positive charge and it then migrated to the 
 cathode. The following table shows the usual 
 charge and migration tendency of a number 
 of aqueous colloidal solutions. 
 
 Charged + Charged 
 
 Migrate to Cathode (- Pole) Migrate to Anode (4- Pole) 
 
 1. Hydrates of Fe, Cu, Cd, Al, Zr, 1. Sulphids of As, Sb, Cu, Pb, Gd. 
 
 Ce, Th. Halides of Ag. 
 
 2. Titanic acid. 2. Stannic acid, silicic acid. 
 
 3. Colloidal Bi, Pb, Fe and Gu 3. Colloidal Pt, Au, Ag, and Hg, 
 
 (Bredig's method). I, S, Se. 
 
 4. Albumen, hemoglobin, agar. 4. Gum arabic, soluble starch, 
 
 gamboge, mastic, oil emulsion. 
 
 5. Basic Dyes: Methyl violet, 5. Acid Dyes: Eosin, fuchsin, 
 
 Bismarck brown, methylen anilin blue, indigo, soluble 
 
 blue, Hofmann violet. Prussian blue. 
 
 PECTIZATION AND PEPTIZATION. Briefly 
 stated pectization means the coagulation of a 
 colloidal sol, and peptization its redispersion. 
 If a small quantity of an electrolyte is added 
 
32 COLLOID CHEMISTRY 
 
 to a pure ruby red colloidal gold solution, the 
 latter changes to a blue or violet color, and 
 deposits its gold as a fine blackish coagulum 
 or precipitate.* By watching in the ultra- 
 microscope the coagulation of very dilute 
 milk by dilute acid, the individual particles of 
 the colloidal casein may be seen to gather 
 gradually together into groups, whose motion 
 becomes progressively less as their size in- 
 creases, until they are no longer able to stay 
 afloat, and finally coagulate in large grape-like 
 clusters. Hardy believes that the particles of 
 colloids adsorb the oppositely charged ions of 
 electrolytes present; at the isoelectric point 
 (that is when there is no excess either of posi- 
 tive or negative charges on the particles) coag- 
 ulation occurs. If, however, an excess of elec- 
 trolyte be added all at once, the isoelectric 
 point may be passed before coagulation occurs, 
 and the particles acquire a charge opposite to 
 the one they had originally. Under such con- 
 ditions, no coagulation may result. 
 
 * The amount in milligrams of protective colloid just 
 sufficient to prevent the change to violet of 10 cc. of bright 
 red colloidal gold solution by the addition of 1 cc. of a 10 
 per cent solution of NaCl, is called the "gold figure" or "gold 
 number" of the protector. 
 
GENERAL PROPERTIES OF COLLOIDS 33 
 
 Burton epitomizes the difference in action 
 of various electrolytes as follows: "Two 
 remarkable results are evident on comparing 
 the coagulative powers of various electrolytes 
 on colloids of different kinds; first, the coagu- 
 lation depends entirely on the ion bearing a 
 charge of sign opposite to that of the colloidal 
 particle; and, second, with solutions of salts, 
 trivalent ions have, in general, immensely 
 greater coagulative power than divalent ions, 
 and the latter, in turn, much greater than 
 univalent. Acids and alkalis in particular 
 cases act more strongly than the corresponding 
 salts." 
 
 High-tension electric discharges may also 
 effect the coagulation or precipitation of a 
 finely subdivided or dispersed phase; which 
 fact was utilized by Sir Oliver Lodge in dis- 
 pelling fogs, and by Cottrell hi coagulating 
 smelter and similar fumes. 
 
 PEPTIZATION. So strong is the analogy 
 between digestion and colloidal disintegration 
 that Thomas Graham, the father of colloid 
 chemistry, coined the word peptization to 
 express the liquefaction of a gel. He first 
 speaks of the coagulation or pectization of 
 
34 COLLOID CHEMISTRY 
 
 colloids. "The pectization of liquid silicic 
 acid," he states, "and many other liquid 
 colloids is effected by contact with minute 
 quantities of salts in a way which is not under- 
 stood. On the other hand, the gelatinous acid 
 may be again liquefied, and have its energy 
 restored by contact with very moderate 
 amounts of alkali. The latter change is 
 gradual, 1 part of caustic soda, dissolved in 
 10,000 water, liquefying 200 parts of silicic 
 acid (estimated dry) in 60 minutes at 100 
 degrees. Gelatinous stannic acid also is easily 
 liquefied by a small proportion of alkali, even 
 at the ordinary temperature. The alkali, too, 
 after liquefying the gelatinous colloid, may be 
 separated again from it by diffusion into water 
 upon a dialyzer. The solution of these col- 
 loids in such circumstances may be looked 
 upon as analogous to the solution of insoluble 
 organic colloids witnessed in animal digestion, 
 with the difference that the solvent fluid here 
 is not acid but alkaline. Liquid silicic acid 
 may be represented as the 'peptone' of 
 gelatinous silicic acid; and the liquefaction of 
 the latter by a trace of alkali may be spoken of 
 as the peptization of the jelly. The pure 
 
GENERAL PROPERTIES OF COLLOIDS 35 
 
 jellies of alumina, peroxide of iron and titanic 
 acid, prepared by dialysis, are assimilated 
 more closely to albumen, being peptized by 
 minute quantities of hydrochloric acid." 
 
 Peptization is in reality deflocculation, a 
 dispersion of groups into separate particles 
 which once more acquire active motion and 
 remain afloat or in solution. The detergent 
 action of soap and dilute alkalis is due to the 
 fact that they deflocculate adhering particles 
 of " dirt." 
 
CHAPTER VI 
 
 Practical Applications of Colloid Chemical 
 Principles 
 
 The practical applications of colloid chemis- 
 try are so manifold and widespread that they 
 touch every branch of science and technology. 
 Whole books may be and have been written on 
 many of the most restricted fields, while the 
 scientific literature teems with monographs 
 and articles, directly on, or applicable to, 
 colloid-chemical subjects. In what follows, it 
 will be possible therefore to give not an ex- 
 haustive, but only a most general survey, 
 intended rather to show the ubiquity of col- 
 loid phenomena; and many important topics 
 must be dismissed with a most rudimentary 
 discussion, altogether incommensurate with 
 theu* importance. 
 
 ASTRONOMY. As matter in colloidal state 
 is so common on our relatively minute earth, it 
 is but natural to expect to find many instances 
 of colloidal dispersion in the immensity of the 
 Universe. 
 
 36 
 
PRACTICAL APPLICATIONS 37 
 
 Cosmic dust Is widely distributed throughout 
 space, and as it is gathered up by the superior 
 attraction of the larger heavenly masses (suns, 
 planets, etc.), which in any system grow at the 
 expense of the smaller masses, fresh quantities 
 are continually produced by the collisions of 
 bodies in space, as well as the disintegration 
 of meteorites, comets, asteroids, etc. 
 
 The tails of comets seem to consist almost 
 entirely, and the nuclei and coma largely, of 
 colloidally dispersed matter. The great comet 
 of 1882 which made a transit of the sun, was 
 invisible against the solar disc (a position corre- 
 sponding to attempted observation of colloidal 
 particles in the ordinary microscope against a 
 luminous background), but became visible 
 again after passing beyond the sun's disc (a 
 position corresponding to successful observa- 
 tion of the same colloidal particles in the ultra- 
 microscope against a dark background, the eye 
 of the observer being protected from the source 
 of illumination). 
 
 The streaming of the cometary tails away 
 from the sun may be due to the ionization of 
 the constituent colloidal particles, and their 
 consequent electrical repulsion; or more prob- 
 
38 COLLOID CHEMISTRY 
 
 ably, it may be due to the sun's rays, as was 
 pointed out by J. Clerk Maxwell. The inten- 
 sity of the action of the sun's rays on a particle 
 depends upon its surface, which varies as the 
 square of its diameter, whereas the gravitation 
 of the same particle to the sun depends upon its 
 mass, which varies as the cube of its diameter. 
 Theoretically in the case of a particle whose 
 density equals that of water, the repulsion 
 balances gravitation when the diameter reaches 
 0.0015 mm. (= 1.5 /*). As the diameter di- 
 minishes the repulsive force gains the ascend- 
 ancy, soon reaching a maximum and again 
 diminishing, until when the particle has a 
 diameter of only 0.00007 (= 70 MM) the two 
 forces again balance each other.* 
 
 These figures, which refer to a substance hav- 
 ing the density of water, are approximately of 
 colloidal dimensions; but in the case of denser 
 bodies the subdivision would be even more 
 profound. It is therefore not surprising that, 
 when the earth recently passed through the tail 
 of a comet, no disturbance of any kind was 
 
 * See Simon Newcomb's article on "Comet," Encyclo- 
 pedia Britannica, llth edition. Also Svante Arrhenius, 
 " Worlds in the Making," Harper & Bros. 
 
PRACTICAL APPLICATIONS 39 
 
 noticed. The comet's tail is a vast celestial 
 camouflage its luminosity a macroscopic Far- 
 aday-Tyndall effect. 
 
 The nebulae, too, apparently consist of finely 
 dispersed matter, rendered luminous by neigh- 
 boring suns; although with them as with the 
 comets, a small part of the light may result 
 from self -luminescence (incandescent gas, etc.). 
 
 METEOROLOGY. What we commonly call 
 " weather conditions " are largely dependent 
 upon the degree of dispersion of water in the 
 atmosphere, and this dispersion is mainly 
 effected and maintained by solar heat and 
 electrical energy. When air carrying water 
 vapor is chilled by rising to a higher level, 
 meeting a colder mass of air, or even by the 
 alternation of night and day, the moisture 
 it contains assumes the colloidal state as cloud, 
 fog or mist; and as the coagulation of the 
 dispersed water proceeds, these in turn may 
 condense still further into dew, rain, snow or 
 hail, depending upon conditions. When the 
 dispersed water aggregates, there is naturally 
 set free the energy originally used in its disper- 
 sion, and this may appear as electricity (light- 
 ning) especially if the aggregation occurs 
 
4:0 COLLOID CHEMISTRY 
 
 suddenly as is the case in thunder and hail 
 storms. We have all noticed how a nearby 
 lightning flash is promptly followed by an 
 increased fall of raindrops. 
 
 Were it not for our atmosphere, the sun would 
 appear to us like a fiery ball set in a black star- 
 sprinkled sky. The blue color of the sky is due 
 to diffraction of the sunlight by the earth's 
 atmosphere, a gigantic Tyndall effect. If we 
 look edgewise through a clear sheet of glass, we 
 at once notice the green color due to colloidally 
 dispersed iron, and in like manner, if we look 
 through a great length of the atmosphere the 
 prevailing color is blue. As the poet Campbell 
 beautifully puts it : 
 
 !< Tis distance lends enchantment to the view, 
 And robes the mountain in its azure hue." 
 
 After the tremendous explosive eruption of 
 the volcano Krakatoa in 1883, colloidal dust 
 and ashes were projected so high that they 
 gradually spread around the earth, causing 
 " golden sunsets." 
 
 We do not know to what extent electrical 
 conditions on the earth affect the dispersion of 
 substances in its atmosphere; but since half 
 of the earth is always heated by the sun while 
 
PRACTICAL APPLICATIONS 41 
 
 the other half is cooler, thermoelectric currents 
 are continually circulating about the earth. 
 Variations in solar radiation due to sun-spots 
 and the like, cause violent electric and magnetic 
 storms which are intimately connected with the 
 aurora, and other atmospheric phenomena 
 (ionization, electrical charge of dispersed par- 
 ticles); and it is well known that sun-spots 
 exercise a potent influence on the weather. 
 
 GEOLOGY AND MINERALOGY. - - The ordi- 
 nary properties of the solid constituents of the 
 earth's crust depend more upon their state of 
 physical subdivision than upon their chemical 
 constitution. Atterberg classified the frag- 
 ments of minerals and rocks as follows: 
 
 Diameter. 
 
 Boulders 2 m. to 20 cm. 
 
 Pebbles 20 cm. to 2 cm. 
 
 Gravel 2 cm. to 2 mm. 
 
 Sand 2 mm. to 0.2 mm. 
 
 Earth 0.2 mm. to 0.02 mm. 
 
 Loam 0.2 mm. to 0.002 mm. 
 
 Clay smaller than 0.002 mm. 
 
 The smaller the particles, the greater their 
 capillarity and the ease with which they are 
 moved by wind and by water, but the less 
 their permeability to water. Fine defloccu- 
 
42 COLLOID CHEMISTRY 
 
 lated clay is carried thousands of miles by 
 rivers until it is finally coagulated by the salts 
 of the ocean, as may be observed in the deltas 
 of the Ganges, Nile and Mississippi. Fine 
 particles are easily cemented by pressure or 
 igneous action into rocks (e.g., sandstone, 
 slate), or may act as a cement for large par- 
 ticles (e.g., pudding-stone) or as a matrix for 
 fossils. 
 
 Many minerals are themselves colloidal gels 
 (e.g., opal, flint, bauxite) or result from the 
 weathering of other minerals with consequent 
 gel formation (e.g., kaolin from kaolinite, 
 serpentine from diabase). Most gems owe 
 their colors to impurities colloidally dispersed 
 within them (e.g., ruby, emerald, amethyst). 
 Dendrites are formed by solutions diffusing 
 through mineral gels. Colloidal minerals usu- 
 ally adsorb, and are dyed by aniline dyes 
 (methylen blue), whereas crystalloid minerals 
 are unaffected. 
 
 CLAY AND CEEAMICS. The effect of vege- 
 table extractive matters on the working 
 properties of clay have been known from 
 ancient times in the Bible (Exodus V) it 
 is mentioned that brick cannot be made with- 
 
PRACTICAL APPLICATIONS 43 
 
 out straw. Recently patents have been taken 
 out for " Egyptianizing " clay by adding to it 
 tannin, extract of straw, humus and the like. 
 Glue and similar protective colloids defloccu- 
 late or "free out " clay and make it " cover " 
 in paper-coating and kalsomining. The work- 
 ing properties of clays depend largely upon the 
 size of their constituent particles and their 
 state of aggregation. This is especially evi- 
 dent in ceramics. Articles molded of clay and 
 then burned, lose their hydrosol condition and 
 become hardened into pottery. 
 
 AGRICULTURE. Although from time im- 
 memorial farmers have classified soils on the 
 basis of their physical and physiological 
 character as "light " or "heavy," "rich " or 
 "poor," "productive" or "unproductive," 
 etc., it is only within comparatively recent 
 years that chemists have begun to realize the 
 full importance of the role played by the 
 colloids, especially the organic colloids of the 
 soil. 
 
 Many important properties of soils, such as 
 permeability, capillarity, absorption, moisture 
 content, etc., are dependent not so much upon 
 the chemical composition as upon the size of 
 
44 COLLOID CHEMISTRY 
 
 the constituent soil particles. (See Atterberg, 
 Schwed, landw. Akad., 1903, and Chem. Zeit.. 
 1905, 29, 195; Patten and Waggaman, U. S. 
 Dept. of Agri. Bureau of Soils, Bull. No. 52, 
 1908). In coarse sand, for example, the 
 amount of water is greatest at the bottom and 
 smallest at the top, whereas in fine clay the 
 distribution is much more uniform. 
 
 Among the natural agencies tending to 
 increase the size of the minute soil particles 
 may be mentioned heat with its drying or 
 evaporative effect, freezing, and the coagulat- 
 ing or flocculating action of soluble inorganic 
 salts and some organic substances present in 
 the soil. On the other hand, included in that 
 little known class of substances vaguely de- 
 scribed as " humus," there are numerous 
 organic substances derived from the bacterial, 
 plant, or animal debris, or exuded by the roots 
 of plants, which act as protective colloids 
 (Schutzkolloide) and tend to produce and 
 maintain the hydrosol, or deflocculatd con- 
 dition. (See P. Ehrenberg, "Die Kollide des 
 Ackerbodens," Zeits. angew. Chem., 1908, 
 41, 2122.) In an excellent paper on the 
 mechanics of soil moisture, L. J. Briggs (U. S. 
 
PRACTICAL APPLICATIONS 45 
 
 Dept. of Agric., Bureau of Soils, Bull. No. 
 10, 1897) pointed out that very small quantities 
 of certain organic substances, such as are con- 
 tinually being produced in the soil by the 
 decay of organic matter, greatly decrease the 
 surface tension of solutions, thus counteracting 
 to a large extent the effects of the surface 
 application of soluble salts which would tend 
 to draw moisture to the surface by increasing 
 the surface tension of the capillary water of 
 soils. It is well known, however, that an 
 excess of salts will ruin a soil physically, as is 
 evident after flooding by sea water or the 
 excessive application of chemical fertilizers. 
 Of interest in this connection is the recent work 
 of the Bureau of Soils, U. S. Department 
 of Agriculture, carried out by Cameron, 
 Schreiner, Livingston and their co-workers. 
 Thus plants grown in the unproductive Ta- 
 koma soil, were greatly benefitted by green 
 manure, oak leaves, tannin and pyrogallol. 
 The injurious effects of quinone and some 
 other organic substances may be due to their 
 ability to precipitate or flocculate the pro- 
 tective colloids of the soil; for as Lumiere 
 and Seyewetz have shown (Bull. Soc. Chim., 
 
46 COLLOID CHEMISTRY 
 
 1907, 4, 428-431; J. S. C. L, 1907, 703) 
 quinone renders gelatin insoluble. 
 
 The fact observed by Fickenday (J. Landw., 
 1906, 64, 343) that more alkali is required to 
 flocculate natural clay soils than kaolin sus- 
 pensions, he attributes to the protective action 
 of the humus present (see Keppeler and Spang- 
 enberg, J. Landw., 1907, 55, 299). 
 
 A. S. Cushman, in his excellent work upon 
 the use of feldspathic rock as fertilizer (U. S. 
 Dept. of Agriculture, Bureau of Plant Indus- 
 try, Bulletin No. 104; Cushman and Hubbard, 
 J. Am. Chem. Soc., 30, 779), has shown that 
 the fine grinding of feldspar increases the 
 amount of potash available under the action of 
 water. Thus, a coarse powder having an 
 area of 43 sq. cm. per cc. of solid feldspar 
 yielded 0.013 per cent, whereas a fine powder 
 whose area was 501,486 sq. cm. per cc. yielded 
 0.873 per cent of potash and soda. These fine 
 particles averaged about 0.1 /* in diameter, 
 which is relatively large as compared with 
 colloidal dimensions; but under the action of 
 physical and chemical soil agencies they 
 undergo further disintegration, finally reaching 
 a colloidal condition in which still more of 
 
PRACTICAL APPLICATIONS 47 
 
 their potash is available, a condition favored 
 and maintained by the organic protective 
 colloids of the soil. 
 
 With these brief and inadequate remarks we 
 must dismiss this subject of such vast impor- 
 tance and fascinating interest, referring to the 
 extensive literature, much of which is quoted 
 in Bulletin No. 52 and the other publications 
 of the Bureau of Soils. 
 
 ELECTROPLATING AND ELECTRODEPOSITION OF 
 METALS. The addition of protective colloids 
 to electroplating baths tends to the production 
 of fine-grained non-crystalline deposits. A. G. 
 Betts in a paper entitled "The Phenomena of 
 Metal Depositing " (J. Am. Electrochem. Soc., 
 
 1905, 8, 63) has shown that there are many 
 factors influencing the action of the colloid, 
 and has suggested a number of possible ex- 
 planations. The correct explanation, how- 
 ever, has been given by Mtiller and Bahntje 
 (Z. Elektrochem., 1906, 12, 317; J. S. C. I., 
 
 1906, 484) who state that the added colloid 
 keeps the deposited metal (copper) in an 
 amorphous, non-crystalline condition, gelatin 
 producing the most powerful effect, egg al- 
 bumen considerably less, while gum and 
 
48 COLLOID CHEMISTRY 
 
 starch have comparatively little action. They 
 also found that the deposited copper weighed 
 about 0.2 per cent more than under normal 
 conditions, indicating that some of the colloid 
 had been carried down with the metal. 
 
 The relative efficiency of the colloids just 
 referred to corresponds to their relative effi- 
 ciency in protecting from coagulation solutions 
 of colloidal gold (see Zsigmondy, J. S. C. I., 
 1902, 192; also Colloids and the Ultramicro- 
 scope, p. 81), which is additional evidence 
 that we have another instance of protective 
 colloidal action, by which the crystallization 
 forces of the metal are powerfully influenced. 
 
 METALLURGY. Since coarsely crystalline 
 metals are brittle, tending to split along the 
 lines of crystal cleavage, various physical and 
 chemical means are employed in technical 
 practice to obtain a hard, fine-grained struct- 
 ure. (See I. Langmuir, Iron & Steel Inst., 
 Sept. 1907; J. S. C. I., 1907, 1094.) Among 
 the physical methods are chilling and rolling, 
 while the chemical methods involve the re- 
 moval of undesirable constituents (as in the 
 conversion of pig iron into steel) or the addi- 
 tion of desirable constituents (as in the case- 
 
PRACTICAL APPLICATIONS 49 
 
 hardening and the manufacturing of " chrome 
 steel/' " nickel steel/ 7 etc.). For example, 
 P. Putz has shown (J. S. C. L, 1907, 614) that 
 the predominant effect of vanadium in steel is 
 to decrease the size of the ferrite grains and 
 make the material tougher; it renders the 
 ordinary structure due to pearlite fine-grained 
 and homogeneous (see also Beilby, Proc. Roy. 
 Soc. A., 79, 463; J. S. C. L, 1907, 926). 
 
 Now, while the question is one of very great 
 complexity, many of the facts at present 
 available seem to indicate that one of the 
 causes favoring the fine-grained structure is 
 the inhibition of crystallization by substances 
 colloidally dissolved in the molten mass. Thus 
 part of the carbon in iron and steel exists in the 
 graphitic form, and as graphite is slightly soluble 
 in iron (see C. Benedicks, Metallurgie, 1908, 
 5, 41; J. S. C. L, 1908, 406); some of it will, 
 under proper conditions, be found in colloidal 
 form (Carnegie Research Reports, J. S. C. L, 
 1908, 27, 570; F. Wust, J. S. C. L, 1907, 26, 
 412; Hersey, J. S. C. L, 27, 531). Besides 
 metals may dissolve each other and other 
 substances colloidally, but in the case of ordi- 
 nary metals this is not easy to demonstrate. 
 
50 COLLOID CHEMISTRY 
 
 An observation recently made by J. Alex- 
 ander * is of interest here. Moissan (Comptes 
 rend., 144, 593, J. S. C. L, 1907, 413) has noted 
 that the addition of a little platinum to me- 
 tallic mercury causes the latter to " emulsify " 
 in water. Upon making up such an " emul- 
 sion," Alexander noticed that the supernatant 
 fluid remained turbid upon standing, and 
 therefore examined the fluid in the ultrami- 
 croscope, which revealed the presence of col- 
 loidal metallic particles in active motion. 
 
 DYEING. The difference between a physi- 
 cal mixture and a chemical compound is 
 frequently illustrated by dissolving out the 
 sulphur from a mixture of iron filings and 
 sulphur dust, and showing that the solvent, 
 carbon bisulphid, does not affect the compound, 
 ferrous sulphid. That in many cases dyeing 
 is due, not to chemical combination, but to an 
 adsorption f of the dye by the colloidal fiber, is 
 evident from the fact that some dyestuffs can 
 be extracted from the dyed fiber by means of 
 alcohol. Investigation has shown that many 
 
 * J. S. C. I., 1909, 28, 280. 
 
 t In some cases adsorption may be followed by undoubted 
 chemical combination. 
 
PRACTICAL APPLICATIONS 51 
 
 dyes are colloidal in solution, and the selective 
 coloring of various fibers, tissues, cells, nuclei, 
 etc., is probably due to selective adsorption or 
 precipitation of one colloid by another. The 
 ultramicroscopic researches of N. Gaidukov 
 (Zeitsch. f. angew Chem., 21, 393) support this 
 view. 
 
 The phenomena of dyeing are rather numer- 
 ous and complicated, for the dyestuffs are 
 numbered by thousands, and the various fibers, 
 tissues, etc., such as cotton, silk, wool, linen, 
 jute and straw, all react characteristically. In 
 some cases the colloid fiber adsorbs the dye, as 
 with basic colors which dye silk and wool 
 directly; in other cases there is necessary a 
 mordant which is first adsorbed and then fixes 
 the color. Certain colors mutually precipitate 
 each other and may in fact serve as mordants 
 for each other, e.g., methylen blue and dianil 
 blue 2 R.; patent blue V and magenta. 
 
 Colloid chemistry also throws much light 
 upon many obscure points in the practical art 
 of dyeing. It is possible to obtain much more 
 level colors in old dye liquors than in fresh 
 ones, and here it seems that colloidally dis- 
 solved substances are responsible, exercising 
 
52 COLLOID CHEMISTRY 
 
 a restraining action upon the absorption of the 
 color. The addition of Glaubers' salt facili- 
 tates level dyeing, probably by its action as an 
 electrolyte, producing a partial coagulation of 
 the dyestuff, so that the particles of the latter, 
 thereby made larger, are absorbed more slowly 
 and evenly. 
 
 SOAP. In a comprehensive paper entitled 
 " Modern views on the constitution of soap " 
 (see J. S. C. L, 1907, 26, 590) Lewkowitsch 
 epitomizes the views of Merklen substantially 
 as follows: " Commercial soap is a product 
 having an essentially variable composition 
 dependent upon (1) the nature of the fatty 
 acids, (2) the composition of the 'nigre' (in 
 the case of settled soaps), (3) the tempera- 
 ture at which the boiling is conducted; it 
 behaves like a colloid and should not be re- 
 garded as a compound of sodium salts of fatty 
 acids, with which a definite amount of water is 
 combined chemically, but rather as an 'ab- 
 sorption-product' whose composition is a func- 
 tion of the environment in which the salts of 
 the fatty acids happen to be at the moment of 
 the finishing operations. " 
 
 Merklen's views conflict with the views as to 
 
PRACTICAL APPLICATIONS 53 
 
 the chemical composition of soap previously 
 advanced by Lewkowitsch, who states, in 
 conclusion: "But whatever may be the out- 
 come of renewed experiments, Merklen's views 
 cannot fail to stimulate further research into 
 the composition of soap, and thus help to raise 
 the industry of soap-making, which has too 
 long been looked upon as a mere art, to the 
 rank of a scientifically well-founded industry, 
 the operations of which are governed by the 
 laws of mass action, the phase rule and the 
 modern chemistry of colloids. " 
 
 The colloidal nature of soap solutions is 
 indicated by their turbidity and their gelatin- 
 ization. That the detergent action of soap is 
 consequent upon its deflocculating effect was 
 brought out in the interesting Cantor Lecture 
 of H. Jackson (J. Soc. Arts, 55, 1101 et seq.), 
 who examined microscopically the supernatant 
 fluid resulting from washing a dirty cloth with 
 soap and water, and found in it countless 
 particles in a state of oscillatory motion 
 ("pedesis ") When an individual fiber was 
 bathed in soap solution, the dirt particles 
 gradually loosened and began to oscillate; 
 upon substituting salt solution for the soap, 
 
54 COLLOID CHEMISTRY 
 
 the particles flocculated and the motion 
 ceased. An ultramicroscopic examination of 
 the detergent effects produced by soap should 
 prove of interest. 
 
 In this connection mention must be made 
 of the excellent paper of W. D. Richardson 
 on "Transparent Soap" (J. Amer. Chem. 
 Soc., 30, 414), which he terms a supercooled 
 or supersaturated solution, having distinctly 
 crystalline tendencies and exhibiting colloidal 
 properties. Having in mind the fact that 
 the salts of the higher fatty acids dissolve 
 in water as colloids, and in alcohol as crystal- 
 loids (S. Ya. Levites, Zeits. Chem. Ind. Kol- 
 loide, 2, 208, et seq., J. S. C. L, 1908, 1134; 
 Mayer, Schaeffer, and Terroine, Compt. rend., 
 146, 484) and also the fact that the alcohol or 
 equivalent solvents (glycerol, sugar, etc.) are 
 used in transparent soap, it seems probable 
 that the crystals which frequently form in it 
 are due to the slow separation of such part of 
 the soap as is in crystalloid solution. This 
 view is supported by the fact adduced by 
 Richardson (loc. cit., p. 418) that the fatty 
 acids separated from the crystals had a higher 
 melting point than those separated from the 
 
PRACTICAL APPLICATIONS 55 
 
 clear matrix. The isolation of the crystals 
 was difficult because of their ramifying tend- 
 ency, which recalls some of the crystal figures 
 exhibited by some mixtures of crystalloids and 
 colloids. What may be called the crystalloid 
 phase of soap is apparently governed by the 
 same factors as those which Tamman has 
 pointed out as governing the crystallization of 
 supercooled solutions, i.e., 1st, the specific power 
 of crystallization; 2nd, the speed of crystalliza- 
 tion; 3rd, the viscosity (see Zsigmondy, Colloids 
 and the Ultramicroscope, p. 128 et seq.). Thus, 
 gold ruby glass when quickly cooled (or super- 
 cooled) is colorless, but acquires a red color 
 upon reheating to the softening point. By 
 ultramicroscopic examination Zsigmondy 
 showed that the nuclei of metallic gold, which 
 in the colorless glass were amicroscopic, grew 
 into ultramicroscopic visibility in the red glass. 
 It therefore seemed to the author that a most 
 important factor in determining the trans- 
 parency of transparent soap would be the 
 speed of cooling, and some experiments were 
 made along this line. 
 
 A piece of commercial transparent soap was 
 melted and cast into two cups, one of which 
 
56 COLLOID CHEMISTRY 
 
 was quickly chilled in ice, while the other was 
 allowed to cool slowly by immersion in hot 
 water. The quickly cooled piece was trans- 
 parent, while the other was practically opaque, 
 and showed upon ultramicroscopic examina- 
 tion much larger ultramicrons than the trans- 
 parent piece. 
 
 After standing three or four months, the 
 quickly cooled soap was still transparent to 
 the naked eye, whereas large opaque spots 
 could be seen in the slowly cooled piece. In 
 the ultramicroscope the former appeared as 
 before, whereas the latter showed large and 
 perfectly resolvable crystals in a clear matrix. 
 
 These experiments give us an inkling as to 
 what occurs during the " heat treatment " and 
 tempering of metals, and it is to be hoped that 
 some technique may be devised that will give 
 us even a clearer insight than does " etching," 
 into the changes that occur in metals in metal- 
 lurgical operations (heat treatment), use, age, 
 and even " disease " (tin for example). 
 
 MILK. From a colloid chemical stand- 
 point, the main constituents of milk may be 
 classified as follows: 
 
PRACTICAL APPLICATIONS 
 
 57 
 
 In crystalloid 
 dispersion 
 
 salts (such as NaCl, etc.) 
 sugar (lactose). 
 
 In colloidal C casein an unstable or irreversible colloid. 
 dispersion ( lactalbumin a stable or reversible colloid. 
 In suspension* milk fat. 
 
 Most formulas and recipes for modifying 
 cows' milk for infant feeding, and for that 
 matter, many analyses, combine the percent- 
 ages of lactalbumin and of casein under the 
 collective title of " total proteids," thereby 
 obscuring the highly important fact that the 
 lactalbumin stabilizes and protects the casein 
 from coagulation by acid and rennin.f 
 
 The subjoined table will show how milks 
 are influenced by a difference in the ratio 
 between the casein and lactalbumin. 
 
 AVERAGE COMPOSITION 
 
 
 
 Lact- 
 
 
 
 
 Kind of milk. 
 
 Casein. 
 
 albu- 
 
 Fat. 
 
 Behavior with 
 acid. 
 
 Behavior with 
 rennin. 
 
 
 
 min. 
 
 
 
 
 Cow 
 
 3 02 
 
 53 
 
 3 64 
 
 Readily coag- 
 
 Readily 
 
 
 
 
 
 ulates. 
 
 coagulates. 
 
 Woman 
 
 1.03 
 
 1 26 
 
 3 78 
 
 Not readily 
 
 Not readily 
 
 
 
 
 
 coagulated. 
 
 coagulated. 
 
 Ass 
 
 67 
 
 1 55 
 
 1 64 
 
 
 
 
 
 
 
 
 
 * It is probable that some of the fat is in colloidal dispersion, 
 t See Alexander and Bullowa, Jour. Am. Med. Assoc., Vol. 
 LV, p. 1196 (Oct. 1, 1910). 
 
58 COLLOID CHEMISTRY 
 
 It is interesting to note that the milks in the 
 above table are arranged in order of their 
 digestibility, which also corresponds with their 
 relative colloidal protection. Thus Jacobi has 
 stated that asses' milk has always been 
 recognized as a refuge in digestive disorders in 
 which neither mother's milk nor cow's milk or 
 mixtures were tolerated. 
 
 The addition of protective colloids to cows' 
 milk stabilizes it and makes it act more like 
 mother's milk when treated with acid and 
 rennin. In fact, if sufficient protective colloid 
 be added, coagulation of the casein in the 
 stomach may be entirely prevented, or at least 
 the coagula kept in a very fine state of sub- 
 division. 
 
 The action of protective colloids is beauti- 
 fully illustrated in the ultramicroscope, which 
 enables us to see the individual particles of 
 cows' casein in active motion and watch the 
 course of their coagulation by acid, first into 
 small and then into larger and larger groups, 
 whose motion decreases as their size increases, 
 until finally they sink out of solution in coagu- 
 lated masses. If, however, some gelatin or 
 gum arabic solution be added to the cows' 
 
PRACTICAL APPLICATIONS 59 
 
 milk before the addition of the acid, the casein 
 particles continue their active dance and do 
 not coagulate. In this connection it is in- 
 teresting to note that the casein particles in 
 mother's milk appear to be much smaller than 
 those in cow's milk, probably because of the 
 more highly protective medium in which they 
 are formed and exist. 
 
 Although their method of action was not 
 perfectly understood, protective colloidal sub- 
 stances have for years been used in the modi- 
 fication of cow's milk for infants. For over 
 thirty years Jacobi has advocated the addi- 
 tion of gelatin and gum arabic to cow's milk 
 and infant's diet, and the use of gruels, dex- 
 trinized starch and similar reversible colloids 
 is familiar to all. It is interesting to note that 
 sodium citrate, which is largely employed as 
 an addition to cow's milk, acts as a protective 
 colloid, and when going into solution actually 
 exhibits actively moving ultramicrons in the 
 ultramicroscope, a fact which indicates its 
 colloidal condition. 
 
 In addition to stabilizing the casein, pro- 
 tective colloids in milk have a very important 
 influence on the milk fat. In the first place 
 
60 COLLOID CHEMISTRY 
 
 is to be considered the emulsifying and 
 emulsostatic action of reversible colloids. Of 
 much greater importance, however, is the 
 result of stabilizing the casein, for insufficiently 
 protected casein in curding carries down 
 mechanically most of the milk fat present, 
 yielding a greasy, fatty curd which is very 
 difficult for the digestive juices to dissolve. 
 
 ICE CREAM. It is a fact well known to 
 practical ice cream makers, and amply proven 
 by experience, that ice cream made without 
 eggs, gelatin or some similar colloidal ingredi- 
 ent, is gritty, grainy or sandy, or else soon 
 becomes so upon standing; whereas ice cream 
 made with small quantities of colloids possesses 
 that rich, mellow, velvety texture so much in 
 demand. Here the added colloid acts as an 
 inhibitor of crystallization or practically speak- 
 ing as a preserver of texture. The added 
 colloid, especially gelatin, which is the one 
 most frequently used, also serves as a protective 
 colloid in preventing the coagulation of casein, 
 apparently an irreversible hydrosol and a 
 normal constituent of ice cream. In view of 
 what has been said above, it is evident that 
 gelatin thus renders ice cream more digestible. 
 
PRACTICAL APPLICATIONS 61 
 
 A very misleading impression is given by 
 some official food chemists referring to gelatin 
 in ice cream as a " filler," which naturally leads 
 to the idea that it is an inferior ingredient 
 added in quantity to cheapen the product. 
 But as gelatin is expensive and as but | per 
 cent is used, such a view is evidently erroneous. 
 The food value of gelatin as a protector of the 
 body's nitrogen being generally admitted, and 
 its effect in milk being very beneficial from a 
 digestive point of view, its use in ice cream in 
 the quantities referred to is necessary, legiti- 
 mate and scientific. 
 
 CONFECTIONARY. In gum drops, marsh- 
 mallows, " moonshine " and other candies, use 
 is made of gum arabic, gelatin, albumen, and 
 other colloids to prevent the crystallization of 
 the sugar. Thus, besides adding to the food 
 value, they give the candy a smooth and 
 agreeable taste, and preserve it in saleable 
 condition. 
 
 BREWING. Beer contains dextrin and al- 
 bumin, both colloids. In the brewing process 
 many factors appear which tend to coagulate 
 the albumen. The influence of solid surfaces 
 is illustrated by changing the walls of the 
 
62 COLLOID CHEMISTRY 
 
 fermenting vessel. Thus a certain wort fer- 
 mented in glass or enameled vessels showed 
 0.2450 per cent of albumen; the same wort 
 fermented in a paraffin-lined vessel showed 
 0.1925, and in a vessel lined with pitch only 
 0.1750 per cent of albumen. Old-fashioned 
 brewers would never use any vessel unless it 
 had first been treated with a decoction of 
 malt kernels and nut leaves, or else with 
 "fassgelager " (barrel dregs) which acts like 
 the so-called "bierstein," a deposit consisting 
 chiefly of organic substances that forms upon 
 new surfaces and protects albumen from 
 coagulation by their influences. 
 
 The influence of fluid surfaces is evident 
 from the fact that in the chemical analysis of 
 beer, benzine, benzol, chloroform, etc., may 
 be used to coagulate and shake out the beer 
 colloids. 
 
 The formation of gas bubbles tends to 
 coagulate the dissolved albumen, and this fact 
 killed the so-called " Vacuum Fermentation 
 Process." The jarring due to transportation 
 or even to passing trains may have a deleteri- 
 ous effect. A slight trace of acid tends to 
 stabilize the albumen as do the tannin and 
 
PRACTICAL APPLICATIONS 63 
 
 resins from the hops, the dextrins from the 
 mash and the inorganic colloids of calcium and 
 magnesium. A proper balance between the 
 dextrin and albumen is necessary for the 
 formation of a lasting foam and a desirable 
 "body" (Vollmundigkeit). 
 
 In America where beer is served icy cold, 
 the chilling produces cloudiness, consequent 
 upon a coagulation of albumen. This was 
 cleverly overcome by Wallerstein, who in- 
 troduced a proteolytic enzyme which increases 
 the degree of dispersion of the albumen and 
 thus prevents the clouding. 
 
 TANNING. The skins of animals (hide) 
 constitute an organized colloid jelly, formed of 
 bundles of fine fibrils, about 1 /* in diameter, 
 bound together by a cementing material of 
 similar chemical composition, which is largely 
 removed by the liming and other treatment, 
 preceding the tannage proper. 
 
 When the swollen hide is placed in the acid 
 tannin solution* (tan liquor), the tannin is 
 powerfully adsorbed by the fiber and combines 
 with it to form leather. It is still a moot 
 
 * In alkaline solution both the tannin and thejaide are 
 negatively charged and no tanning occurs. 
 
64 COLLOID CHEMISTRY 
 
 question whether the combination is "phys- 
 ical " or "chemical," but since the fixation of 
 the tannin follows an adsorption isotherm and 
 is reversible in the presence of alkalis, it may 
 justly be called a " colloid combination" 
 which partakes of the nature of both. The 
 positively charged hide and the negatively 
 charged tannin mutually coagulate each other. 
 Gelatin when neutral and free from electro- 
 lytes does not precipitate pure tannin, but in 
 acid solution it takes a positive charge and is 
 tanned. The tanning process may be aided 
 electrically by giving the hide a suitable 
 potential, positive in the case of tannin and 
 negative in the case of chromium compounds. 
 
 RUBBER. Rubber is made by coagulating 
 the milky juice (latex) of various plants. 
 Rubber latices are emulsions stabilized by 
 protective colloids (proteins or peptones) and 
 the nature of the coagulant depends upon the 
 nature of the protector. Thus, formaldehyde 
 preserves latices whose protectors are proteins, 
 but coagulates Kickexia latex by precipitating 
 the protective peptones. 
 
 Vulcanization consists of the combination 
 of sulphur with rubber. At first the sulphur 
 
PRACTICAL APPLICATIONS 65 
 
 is adsorbed; and then by heating, part of it 
 enters into a close combination, probably true 
 chemical combination. 
 
 PHOTOGRAPHY. The photographic plate 
 owes its sensitiveness to an " emulsion" of 
 colloidal silver halides stabilized by a protec- 
 tive colloid (gelatin, albumen or collodion). 
 The degree of dispersion is controlled by the 
 conditions of precipitation of the silver salt 
 and the subsequent treatment of the emulsion 
 (ripening). The latent image formed upon 
 the exposure of the plate to light is probably 
 an adsorption compound between colloidal 
 silver and the silver halides. 
 
 BOILER SCALE. In addition to containing 
 various salts intended to precipitate scale- 
 forming ingredients, most formulas for " boiler- 
 compounds " and scale-preventing mixtures 
 include such substances as glue, dextrin, 
 starch, potatoes, tannin, extract of hemlock, 
 etc. These colloids undoubtedly prevent the 
 formation of hard crystalline scale, either by 
 inhibiting to some extent the precipitation of 
 the scale-forming salts or by keeping the 
 precipitate in an extremely fine non-crystal- 
 line condition. 
 
66 
 
 COLLOID CHEMISTRY 
 
 CEMENT, MORTAR AND PLASTER. When 
 freshly mixed, cement and mortar contain 
 colloidal sols or gels, which gradually coagu- 
 late or "set " and bind the crystalline elements 
 of the plaster into a coherent whole. 
 
 The setting of the plaster of Paris is delayed 
 by glues, gums and other colloidal substances, 
 and "retarders " of this character have been 
 in use for years. On preparing some micro- 
 scope slides with a mixture containing equal 
 parts of plaster of Paris and water, to which 
 had been added varying proportions of gelatin, 
 the following results were observed: 
 
 Per cent 
 gelatin. 
 
 Time to set 
 in minutes. 
 
 Microscopic appearance of slide. 
 
 
 
 40 
 
 Characteristic interlacing crystals of calcium 
 
 
 
 sulphate. 
 
 T& 
 
 50 
 
 No true crystals except in a few spots, where 
 
 
 
 some colloid-free solution had diffused out. 
 
 
 
 Elsewhere aborted sphero-crystals. 
 
 & 
 
 260 
 
 No true crystals. 
 
 1 
 
 910 
 
 No true crystals. 
 
 * 
 
 960 
 
 No true crystals. 
 
 1 
 
 Not set in 
 
 No true crystals. 
 
 
 48 hours. 
 
 
 2 
 
 Not set in 
 
 No true crystals. 
 
 
 48 hours. 
 
 
 FILTRATION. Successful filtration depends 
 upon the use of a septum or filtering medium, 
 
PRACTICAL APPLICATIONS 67 
 
 whose pores or orifices are small enough to 
 hold back the particles it is desired to sepa- 
 rate from the fluid; or the pores may be- 
 come small enough by the deposit upon or in 
 them of some of the precipitate, or of some 
 added material, such as paper pulp, kieselguhr 
 or shredded asbestos. It is, therefore, evident 
 that the presence of protective colloids, by 
 tending to produce the finely dispersed or 
 "hydrosol" condition of the particles, favors 
 their passage through the filter. Thus a gold 
 hydrosol with particles of 20-30 ju^ and con- 
 taining albumen, passed freely through a 
 Pukall and a Maassen filter. In the absence 
 of the protective albumen, the colloidal gold 
 was adsorbed by the filter, gradually clogging 
 the pores until the filtrate, at first red, became 
 colorless. In technical practice, wherever 
 possible, a coagulated precipitate is formed, 
 whose large particles are held back with com- 
 parative ease. It is very difficult to filter glue 
 or gelatin solutions or precipitates formed in 
 the presence of protective colloids. 
 
 The successful treatment of sewage, back- 
 waters and trade effluents depends largely upon 
 the separation from them of colloidal impuri- 
 
68 COLLOID CHEMISTRY 
 
 ties by coagulation, adsorption and filtration. 
 The old ABC method depended upon the use 
 of alum, blood and clay (whence the name) to 
 make a coagulum which would carry down 
 suspended matter. Ferrous sulphate and lime 
 (yielding a coagulum of ferric hydroxid) and 
 alum are also used as clarifiers and coagulants. 
 Filtration through sand, coke, etc., is made use 
 of to adsorb finely dispersed impurities. 
 
 Animal charcoal and fuller's earth decolorize 
 sugar and oils respectively, because of their 
 powerful adsorptive action. 
 
 CHEMICAL ANALYSIS. The presence of col- 
 loids, especially in technical products or solu- 
 tions, may lead to grave errors in analysis, so 
 that the chemist should destroy them by 
 ignition, or else nullify their effects by the 
 addition of a sufficient excess of coagulant or 
 precipitant. Reversible colloids which are 
 frequently referred to under the vague term 
 " organic matter " may act: (1) by totally or 
 partially preventing the formation of precipi- 
 tates, just as tartaric acid and tartrates prevent 
 the precipitation of alumina, chromic oxid, and 
 ferric oxid (see Yoshimoto, J. S. C. I., 1908, 
 27, 952); (2) by preventing the satisfactory 
 
PRACTICAL APPLICATIONS 69 
 
 filtration of the precipitate formed (see Mooers 
 and Hampton, J. Am. Chem. Soc., 30, 805); (3) 
 by rendering precipitates difficult to wash and 
 purify (see Duclaux, J. S. C. L, 1906, 25, 866). 
 
 A few experiments will serve to make clear 
 the importance of these remarks. Three 
 solutions of lead acetate were taken; to the 
 first was added hydrochloric acid which yielded 
 a heavy coagulated precipitate; to the second 
 was added sodium chlorid (a less highly 
 ionized precipitant) which yielded a colloidal 
 precipitate of lead chlorid; to the third was 
 added, first, a little glue solution and then 
 sodium chlorid which in this case gave no 
 precipitate at all. 
 
 Again in the presence of glue, silver nitrate 
 gives with sodium chlorid only an opalescence 
 which passes through filter paper. Even a 
 large excess of hydrochloric acid fails to pro- 
 duce a precipitate. But upon adding silver ni- 
 trate solution to a chlorid solution containing no 
 colloid, a copious precipitation occurs at once. 
 
 PHARMACY. Colloids, such as gum arabic, 
 Irish moss, tragacanth, etc., are largely used 
 in pharmacy in the preparation of emulsions. 
 If ferric chlorid be added to gum arabic emul- 
 
70 COLLOID CHEMISTRY 
 
 sion of cod liver oil, it coagulates the gum, and 
 the oil, no longer protected by the emulso- 
 static action of the gum, promptly separates 
 out. 
 
 Colloidal silver (collargol, argyrol), colloidal 
 mercury (hygrol, blue ointment), and col- 
 loidal sulphur (ichythol) are largely used 
 medicinely. Ferric salts, especially the chlo- 
 rid which readily hydrolyzes into the hydrate, 
 act as styptics or hemostatics by coagulating 
 the blood colloids. The action of disinfectants 
 is largely controlled by colloid-chemical factors 
 the disinfectants are adsorbed by bacteria, 
 and either coagulate their protoplasm or flock 
 them out. 
 
 FOODS AND THEIR PREPARATION. It is a 
 serious error to judge foods upon the basis of 
 a bald chemical or calorific analysis. Fat, 
 protein, carbohydrate and calories are not 
 alone the criteria of food value the physical 
 condition of food largely governs its useful- 
 ness to the organism. The experiences of 
 centuries has taught us the value of "light" 
 bread or cake, leavened by yeast or baking 
 powder until it presents an enormous surface 
 to the digestive juices; unleavened bread was 
 
PRACTICAL APPLICATIONS 71 
 
 eaten only in time of stress, as we learn from 
 the Bible. The meats yielded by young 
 animals are more juicy and tender than those 
 obtained from older animals, because the latter 
 are formed from tissues partially dehydrated 
 by age. 
 
 The ancient art of cooking involves many 
 factors besides mere digestibility and assimila- 
 tion; taste, flavor, odor and variety are 
 important. Egg albumen when cooked is 
 probably more slowly absorbed and loses its 
 species-specificity; therefore, some people who 
 have an idiosyncracy against raw eggs can 
 eat cooked eggs. Cream is an emulsion of fat 
 in an aqueous medium and wets paper; butter 
 is an emulsion of water in a fatty medium and 
 greases paper. 
 
 PHYSIOLOGY AND PATHOLOGY. The changes 
 which occur on almost all physiological proc- 
 esses are remarkable not only because of 
 their very profound nature, but also because 
 they are produced at comparatively low tem- 
 peratures and in the presence of very dilute 
 reagents. The living organism disintegrates 
 proteins, oxidizes carbohydrates and with the 
 same apparent ease synthesizes substances of 
 
72 COLLOID CHEMISTRY 
 
 great complexity. Powerful reagents and high 
 temperatures, which would be destructive to 
 life, are necessary to bring about changes of 
 this character under ordinary laboratory con- 
 ditions. 
 
 The body and plant colloids (biocolloids) 
 consist of carbohydrates (starch, cellulose, 
 glycogen), proteins (plant and animal al- 
 bumins), and lipoids (lecithin, cholesterin, fats 
 and oils). Each tissue has a normal turgor or 
 state of swelling which is greatly influenced 
 by acids, alkalis and salts. The swelling and 
 shrinking of tissues, together with their selective 
 adsorption and the differential diffusion of 
 solutions through them, account for or accom- 
 pany many physiological phenomena, both 
 normal and pathological. Thus, fibrin and 
 gelatin swell much more in very dilute acid 
 than in distilled water, but the swelling is 
 depressed by salts. Fibrin is so sensitive that 
 it swells in the presence of traces of acid quite 
 undetectable by ordinary indicators, such as 
 litmus; in fact fibrin itself is a most sensitive 
 indicator.* 
 
 * Though the normal H ion concentration of the blood is 
 0.37 X 10~ 7 , a concentration of 1.00 X 10~ 7 nH represents 
 an advanced acid intoxication. 
 
PRACTICAL APPLICATIONS 73 
 
 Local accumulation of acid in the organism 
 may cause swelling (edema) ; for example, in- 
 sect stings, which may be imitated by stinging 
 gelatin with a needle dipped in acid. If acid 
 accumulates in an organ with a rigid capsule 
 (eye or kidney), the swelling tends to establish 
 a vicious circle (glaucoma, nephritis) by com- 
 pressing the blood vessels and cutting down 
 the alkaline blood stream, which is unable to 
 wash out the acids (mainly CO 2 ) formed by 
 living protoplasm. 
 
 If the oxidation processes of the body are 
 normal, the hydrogen in foods is oxidized 
 mainly to water and the carbon mainly to 
 carbonic acid a gaseous acid which is ex- 
 haled without demanding fixed alkali or pro- 
 tein of the organism for its elimination. It 
 would require nearly two pounds of pure caus- 
 tic soda to neutralize the acidity produced daily 
 by an average man. In the case of pathologi- 
 cal oxidation, however, other non-volatile acids 
 are formed and a condition called "acidosis " 
 may arise, which is in reality a diminished 
 alkalinity, recognizable by the fact that an 
 abnormally large quantity of bicarbonate of 
 soda is needed to render the urine alkaline. 
 
74 COLLOID CHEMISTRY 
 
 These acids may cause disturbances of the 
 body colloids, disease and even death. In fact, 
 throughout life there is a gradual syneresis of 
 the biocolloids accompanied by visible shrink- 
 ing and loss of water compare the chubby 
 hand of a child with that of an old man. In 
 plants an analogous process occurs in lignifi- 
 cation. 
 
 DIGESTION. The digestive process is pre- 
 liminary to the actual adsorption and use of 
 food by the organism, and has for its object 
 the modification or change of the ingested food 
 into such forms or such substances as may be 
 absorbed in the lower part of the digestive 
 tube. To have a correct understanding of the 
 absorption of the products of digestion, we 
 must bear in mind the fact that the walls of the 
 digestive tract act as semipermeable colloid 
 membranes and that absorption involves dif- 
 fusion into or through these membranes or 
 their constituent cells. Substances in crystal- 
 loidal solution, and colloidal sols whose par- 
 ticles are sufficiently small, represent then the 
 two classes of digestion products which are 
 diffusible and therefore absorbable. 
 
 Food as ingested consists mainly of sub- 
 
PRACTICAL APPLICATIONS 75 
 
 stances that may be grouped into two classes: 
 
 1. Crystalloids such as water, sugars, 
 sodium chlorid, etc. 
 
 2. Colloids such as starch, proteins, emul- 
 sions, etc. 
 
 The crystalloids in foods are usually absorbed 
 directly, although sucrose, for example, under- 
 goes inversion. The colloids, as a rule, are not 
 directly absorbable, and, for the most part, 
 digestion consists in^the disintegration of the 
 colloidal complexes of the food, so that they 
 can actually diffuse into the organism and 
 there undergo further changes. Colloidal gels 
 or even sols whose particles are of large size 
 are, practically speaking, non-diffusible, and 
 must, therefore, be reduced to a more finely 
 dispersed state. 
 
 Investigation has demonstrated that the 
 high efficiency of the digestive juices is mainly 
 due to small quantities of certain colloidal 
 substances called enzymes (such as ptyalin, 
 pepsin and pancreatin) which act as catalyzers, 
 enormously hastening reactions which would 
 otherwise proceed so slowly that, practically 
 speaking, they would not occur at all. The 
 enzymes appear to act by forming with the 
 
76 COLLOID CHEMISTRY 
 
 substrate a combination of unstable character, 
 which breaks down and liberates the enzyme 
 again to continue the operation. Recently 
 W. M. Bayliss, in his interesting monograph 
 on "The Nature of Enzyme Action," has 
 shown that in all probability "the compound 
 of enzyme and substrate, generally regarded as 
 preliminary to action, is in the nature of a 
 colloidal adsorption compound." Anyone who 
 has seen in the ultramicroscope the extremely 
 active motion of the individual particles in 
 colloidal solutions, can readily imagine the 
 terrific bombardment a substance must un- 
 dergo when a colloid enzyme is concentrated 
 on its surface by adsorption, and indeed it 
 seems probable that enzymes actually produce 
 their effects by virtue of their specific surface 
 actions and the motion of their particles. 
 
 In order to find out if this idea could be 
 verified by actual observation, the author 
 watched under the ultramicroscope the action 
 of diastase upon potato starch grains and the 
 action of pepsin upon coagulated egg albumen. 
 
 In the first case, actively moving ultra- 
 microns in the diastase solution gradually 
 accumulated about the starch grains, which 
 
PRACTICAL APPLICATIONS 77 
 
 after a time showed a ragged and gnawed 
 margin. While the adsorption and motion of 
 the larger ultramicrons was all that could be 
 followed, the bright appearance of the field 
 indicated that more numerous finer particles 
 were present, and some apparently of inter- 
 mediate size were seen. 
 
 For observations on albumen there was used 
 a dilute solution of white of an egg which has 
 been heated nearly to boiling. It was opales- 
 cent and in the ultra apparatus exhibited a field 
 full of bright and rapidly moving ultramicrons. 
 Upon allowing a droplet of essence of pepsin 
 (Fairchild's, containing 15 per cent of alcohol 
 by weight) to diffuse in, an immediate coagu- 
 lation occurred, the particles clumping into 
 very large masses. A droplet of decinormal 
 hydrochloric acid was then allowed to diffuse 
 in, whereupon the large masses broke up in 
 small groups and single ultramicrons, which 
 once more resumed their original motion. 
 Soon, however, the albumen particles began 
 to grow smaller and disappear, the field all the 
 while becoming brighter and brighter, indicat- 
 ing the concommitant appearance of smaller 
 ultramicrons or amicrons. In vitro the addi- 
 
78 COLLOID CHEMISTRY 
 
 tion of the pepsin to the opalescent albumen 
 solution caused it to clear gradually, even at 
 room temperature. 
 
 Enzymes are inactivated to a greater or less 
 extent by shaking, heating, electrolytes, etc., 
 all of which, as is well known, cause the 
 coagulation of colloidal solutions and a result- 
 ing decrease in the activity of the motion of 
 their constituent particles. Another feature 
 of interest is that the action of enzymes is 
 reversible, a fact that does not come much 
 into evidence because of the dilution and 
 removal by diffusion of the products formed. 
 In cells, tissues and organs, however, changes 
 of concentration again occur and synthetic 
 processes may result. 
 
 One principle of colloid chemistry is of the 
 utmost importance in digestion, namely: the 
 protective action of reversible colloids, which 
 stabilize or protect from coagulation irrevers- 
 ible or unstable colloids. Mucin and analo- 
 gous colloidal substances undoubtedly have a 
 function of this character, which may in some 
 cases account for the variance between the 
 action of natural and artificial digestive juices. 
 The effects of colloidal protection are in 
 
PRACTICAL APPLICATIONS 79 
 
 evidence in almost all physiological reactions 
 and processes, and it is indeed extremely 
 doubtful if there ever occurs in vivo any 
 chemical reaction which is not greatly in- 
 fluenced by the colloids always present. 
 
 ABSORPTION, SECRETION AND EXCRETION. 
 These are largely affected by the swelling and 
 shrinking of the body colloids and by selective 
 adsorption and diff erential diffusion. It must 
 be remembered that the blood is in reality a 
 circulating fluid colloid, whose attraction for 
 water is greater in the "acid " or venous con- 
 dition, than it is in the " alkaline " or arterial 
 condition. Tissues and organs well supplied 
 with venous blood tend to adsorb water 
 (intestine); whereas those well supplied with 
 arterial blood tend to give up (secrete or ex- 
 crete) water (kidney); and as the blood is 
 passing in a continuous stream, the process 
 continues as long as the water supply permits 
 and until the blood is in equilibrium with the 
 other tissues.* 
 
 * The functioning of organs is largely controlled by nervous 
 influences. Thus a sudden nervous shock may by vaso-dilation 
 send an excessive supply of arterial blood through the mes- 
 enteric arteries (an "internal blush"), and result in a secre- 
 tion of fluid into the intestine (nervous diarrhea). 
 
80 COLLOID CHEMISTRY 
 
 Conditions which decrease the capacity of 
 the blood and tissues to hold water (diuretics, 
 hyperglucemia and acidosis in diabetes) natu- 
 rally result in the elimination of the excess or 
 "free " water (polyuria, diarrhea). 
 
 Minute quantities of acid increase the swell- 
 ing capacity of colloids, which quickly reaches 
 a maximum; after which increasing acidity 
 causes shrinking. Neutral salts oppose the ac- 
 tion of acids apparently by driving back the 
 ionization of the acid and thereby reducing the 
 H-ion concentration which is the controlling 
 factor. 
 
 The action of selective adsorption and 
 differential diffusion in effecting secretion and 
 excretion must be at once manifest. Easily 
 hydrolyzable compounds may be thus split up 
 in the body, and yield secretions of acid nature 
 like the gastric juice, or of alkaline nature like 
 the pancreatic juice, depending upon the 
 structure of the organ, the location of its 
 cavity and of its afferent and efferent vessels. 
 Individual compounds in the blood stream or 
 other body juices may also be selectively 
 diffused out, concentrated or separated from 
 other accompanying substances. By selective 
 
PRACTICAL APPLICATIONS 
 
 81 
 
 adsorption, circulating substances may be 
 fixed and taken from the circulation; in fact, 
 poisons are usually taken up selectively by 
 certain organs and tissues. 
 
 An insight into the mechanism of body 
 processes may be obtained by considering the 
 
 Convoluted 
 Tubule^ 
 
 Vascular 
 Plexus 
 (Ohmerulus) 
 
 f|l<~7iy/ of Renal Artery 
 FIG. 1. Glomerular structure.* 
 
 functioning of the kidney (see Fig. 1). The 
 Malpighian tufts are plentifully supplied 
 with arterial blood having "free water/' and 
 
 * From Dr. J. G. M. Bullowa's translation of Bechhold's 
 "Colloids in Biology and Medicine," D. Van Nostrand Co., 
 1919. 
 
82 COLLOID CHEMISTRY 
 
 under the pulsating pressure* of the blood 
 stream, they ultrafilter off a very dilute but 
 copious blood ultra-filtrate into the long con- 
 voluted tubules. The tubules, however, are 
 plentifully supplied with venous blood, which 
 is unsaturated with water and which there- 
 fore reabsorbs most of the water together 
 with some of the dissolved substances contained 
 in the preliminary excretion ; so that there drips 
 into the pelvis of the kidney a concentrated 
 urine having in solution many of the substances 
 found hi the blood, but in a much higher 
 concentration. Bechhold estimates that the 
 average of two liters of urine voided daily by an 
 average man, represents a preliminary excretion 
 of fifty liters, of which forty-eight are reab- 
 sorbed within the kidney itself. 
 
 In plants, differential diffusion and selective 
 adsorption seem to be intimately bound up 
 with growth and the circulation of the sap. 
 The plant tissues are mainly colloidal gels or 
 finely integrated structures, and as the sap 
 circulates or diffuses through them, each tissue 
 
 * Since the vas defferens has a smaller lumen than the vas 
 efferens, a " back pressure " is created within the Malpighian 
 tufts. 
 
PRACTICAL APPLICATIONS 83 
 
 selectively adsorbs and elaborates certain 
 particular constituents. Thus with the potato 
 and tapioca plants the starch forming sub- 
 stances are fixed in the roots; with the sago 
 palm they are fixed in the stem pith ; and with 
 cereal grains, in the seeds. As long as the 
 adsorptive tissues are unsaturated or are 
 multiplied, so long can growth continue, the 
 stem and branches taking up the substances 
 required for the upward growth, and the root 
 taking up those required for the downward 
 growth. 
 
 When we consider the great variety of bio- 
 colloids and their susceptibility to changes of 
 structure and diffusive or adsorptive capacity, 
 we can easily understand the almost infinite 
 number of reactions that may go on within 
 their recesses, as they swing the balance of the 
 law of mass action over particles reduced to 
 a reactive degree of subdivision. 
 
BIBLIOGRAPHY 
 
 The following are some of the more important standards of 
 reference: 
 
 ENGLISH 
 
 H. BECHHOLD, "Colloids in Biology and Medicine" (trans. 
 
 by Dr. J. G. M. Bullowa). 1919. 
 
 E. F. BURTON, "The Physical Properties of Colloidal Solu- 
 tions." 1916. 
 M. H. FISCHER, "(Edema and Nephritis." 1915. "Fats 
 
 and Fatty Degeneration." 1917. 
 Wo. OSTWALD, "Theoretical and Applied Colloid Chemistry" 
 
 (trans, by Dr. M. H. Fischer). 1917. 
 Wo. OSTWALD, "An Introduction to Theoretical and Applied 
 
 Chemistry" (trans, by Dr. H. M. Fischer). 1916. 
 ZSIGMONDY, "Colloids and the Ultramicroscope " (trans, by 
 
 J. Alexander). 1909. 
 ZSIGMONDY, "Chemistry of Colloids" (trans, by E. Spear). 
 
 1917. 
 
 FRENCH 
 
 COTTON ET MOUTON, "Les Ultramicroscopes et les objets 
 
 Ultramicroscopiques." 1906. 
 PAUL GASTOU, " L'Ultramicroscope dans le Diagnostic Cli- 
 
 nique et les Recherches de Laboratoire." 1916. 
 PERRIN, Numerous Journal Articles. 
 
 85 
 
86 BIBLIOGRAPHY 
 
 GERMAN 
 
 ARTHUR IV!ULLER, "Allgemeine Chemie der Kolloide." 1907. 
 Wo. OSTWALD, "Grundriss der Kolloidchemie." 1909. 
 THE SVEDBERG, "Herstellung Kolloider Losungen." 1909. 
 FREUNDLICH, "Kapillarchemie." 1909. 
 VAN BEMMELEN, "Die Absorption." 1911. 
 
 The "Zeitschrift fur Chemie und Industrie der Kolloide 
 (Kolloid-Zeitschrift)" and " Kolloidchemische Beihefte," 
 published by Wo. Ostwald, are mines of information, con- 
 taining both original articles and references. 
 
 Abstracts of, or references to practically all current articles 
 and books on Colloid Chemistry are to be found under the 
 division "Physical Chemistry "of "Chemical Abstracts," 
 published by the American Chemical Society. Furthermore, 
 in the books above referred to, especially Burton, are to be 
 found numerous valuable references. 
 
AUTHOR INDEX 
 
 ALEXANDER, J., 50, 57, 85. 
 ARRHENIUS, S., 38. 
 ATTERBERG, 41, 44. 
 BAHNTJE, 47. 
 BAYLISS, 76. 
 BECHHOLD, 26, 81, 85. 
 BEHRE, 16. 
 BEILBY, 49. 
 BENEDICKS, 49. 
 BETTS, A., 47. 
 
 BlLTZ, 16. 
 
 BREDIG, 10. 
 BULLOWA, 57, 81, 85. 
 
 BUNSEN, 16. 
 
 BURTON, 33, 85. 
 CAMERON, 45. 
 COEHN, 30. 
 COTTON, 23, 85. 
 COTTRELL, 33. 
 
 CUSHMAN, 46. 
 
 DE BRUYN, LOBRY, 8. 
 DUCLAUX, 69. 
 EHRENBERG, 44. 
 
 FlCKENDAY, 46. 
 
 FISCHER, M. F., 85. 
 FREUNDLICH, H., 86. 
 GAIDUKOV, 51. 
 GASTOU, P., 85. 
 GRAHAM, T., 1, 25, 33. 
 HAMPTON, 69. 
 HARDY, 10, 31, 32. 
 HERSEY, 49. 
 HUBBARD, 46. 
 IGNATOWSKI, 23. 
 JACKSON, H., 53. 
 KEPPELER, 46. 
 LANGMUIR, 48. 
 LEVITES, 54. 
 
 87 
 
 LEWKOWITSCH, 52. 
 LIVINGSTON, 45. 
 LODGE, O., 33. 
 
 LUMIERE, 45. 
 
 MAXWELL, 38. 
 MAYER, 54. 
 MERKLEN, 52. 
 MOOERS, 69. 
 MOUTON, 23, 85. 
 MUELLER, 25, 47, 85. 
 NEWCOMB, SIMON, 38. 
 OSTWALD, WOLFGANG, 10, 14, 
 
 85, 86. 
 PATTEN, 44. 
 PERRIN, 85. 
 PIERONI, 14. 
 PUTZ, 49. 
 RAFFO, M., 14. 
 RAMSEY, 8. 
 
 RICHARDSON, W. D., 54. 
 SCHAEFFER, 54. 
 SCHREINER, 45. 
 SEYEWETZ, 45. 
 
 SlEDENTQPF, 11, 19, 23. 
 
 SPANGENBERG, 46. 
 SPEAR, E., 85. 
 SPRING, W., 14. 
 SVEDBERG, THE, 86. 
 TERROINE, 54. 
 VAN BEMMELEN, 86. 
 VAN CALCAR, 8. 
 VON WEIMARN, P. P., 10. 
 WAGGAMAN, 44. 
 WUST, 49. 
 YOSHIMOTO, 68. 
 ZSIGMONDY, 7, 10, 11, 16, 17, 
 25, 48, 55, 85. 
 
SUBJECT INDEX 
 
 Absorption, 19, 86. 
 
 Acidosis, 73. 
 
 Adsorption, 16, 50, 82. 
 
 Agriculture, 43. 
 
 Amicrons, 22. 
 
 Astronomy, 36. 
 
 Atmosphere, 40. 
 
 Boiler scale, 65. 
 
 Brewing, 61. 
 
 Brownian motion, 8. 
 
 Cardioid Condenser, 23. 
 
 Cement, 66. 
 
 Ceramics, 42. 
 
 Chemical Analysis, 68. 
 
 Clay, 42. 
 
 Coehn's law, 30. 
 
 Colloid Chemistry, Applica- 
 tions of, 36. 
 definition of, 6. 
 
 Colloids, Classification of, 10. 
 
 Colloidal protection, 24, 78. 
 
 Comets, 37. 
 
 Confectionary, 61. 
 
 Cosmic dust, 37. 
 
 Deflocculation, 35. 
 
 Dialysis, 25. 
 
 Diffusion, 25, 27. 
 
 Digestion, 74. 
 
 Dimensions of colloidal par- 
 ticles, 7. 22. 
 
 89 
 
 Dispersoids, 10. 
 
 Dyeing, 50. 
 
 Edema, 73. 
 
 Electric charge of colloidal 
 
 particles, 30. 
 Electrophoresis, 30. 
 Electroplating, 47. 
 Emulsoids, 12. 
 Enzymes, 75, 78. 
 Excretion, 79. 
 Fatty Degeneration, 85. 
 Faraday-Tyndalleffect,20,39. 
 Filtration, 66. 
 Foods, 70. 
 Geology, 41. 
 Glaucoma, 73. 
 Gold number, 32. 
 Hydrophile colloids, 12. 
 Hydrophobe colloids, 12. 
 Ice cream, 60. 
 Irreversible colloids, 10, 12. 
 Isoelectric point, 32. 
 Kidney, 81. 
 Lyophile colloids, 12. 
 Lyophobe colloids, 12. 
 Metallurgy, 48. 
 Meteorology, 39. 
 
 Micron, (M) = YQQQ mm< 
 Migration of Colloids, 31. 
 
90 SUBJECT INDEX 
 
 Milk, 56. Radius of molecular attrac- 
 
 , N 1 tion, 14. 
 
 Millimicron, M = 1^0 * Reversible coUoids, 10. 
 
 1 Rubber, 64. 
 
 ~ 1,000,000 m Schutz kolloide, 44. 
 
 Mineralogy, 41. Secretion, 79. 
 
 Mortar, 66. Soap, 52. 
 
 Nephritis, 73, 85. Soils, 43. 
 
 Pathology, 71. Solution, 7. 
 
 Pectization, 31. Submicrons, 22. 
 
 Pedesis, 8, 53. Suspensions, 7. 
 
 Peptization, 31, 33. Tanning, 63. 
 
 Pharmacy, 69. Tyndall effect, 20. 
 
 Photography, 65. Ultrafiltration, 25. 
 
 Physiology, 71. Ultramicrons, 22. 
 
 Plaster, 66. Ultramicroscope, 17. 
 
 Purple of Cassius, 16- Weather, 39. 
 
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TABLE II 
 
 LjO. 
 
 Suspensio n-s- 
 
 uold Sus- 
 pension G 
 
 Classification of Colloidal Solutions 
 
 according to the size of the particles contained in them and 
 according to their behavior upon desiccation. 
 
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 UNIVERSITY OF CALIFORNIA UBRARY