7 8 PRIXC1PLES OF GENERAL PHYSIOLOGY 



case in point. Urn-, as I rind, tlie temperature coefficient is either :><; <\r 7'.', according 

 to whether the measurements are made from higher to lower, or vice versa. The difference 

 is, no doubt, due to hysteresis (see below). The measurements were made in a quartz vessel. 

 The hypothesis can be tested in another way, not so satisfactory in practice. If a dilute 

 solution of an electrolyte be further diluted, say to twice its volume, its conductivity will be 

 halved, because no new ions will be produced. If the conductivity of a colloidal solution be 

 due to traces of electrolyte impurity, on dilution its conductivity will be reduced in exactly 

 the same proportion. Whereas, if due to slight true solubility of the colloid itself, it will 

 remain unaltered ; or at all events, less diminished than in ratio to the dilution. Tln-r- 

 is always excess of the solid phase present, so that the external phase is always a saturated 

 solution. If the particles diminished in size, owing to further subdivision, greater dispersion, 



milli moles per litre. This fact is in agreement with the experience of chemists that large 

 particles in precipitates grow at the expense of smaller ones ; or from a mixture of crystals, 

 deposited from a hot saturated solution when it cools, the smaller crystals gradually disappear 

 while the larger ones increase in size. The fact is connected with the diminution of surface 

 energy involved in the process. 



Perrin (1905, p. 85) divides colloidal solutions into "hydrophile" and "hydro- 

 phobe," according to the affinity of the dispersed phase for the water ; " lyophile " 

 and "lyophobe" would be better, as Freundlich points out, since water may be 

 replaced by other solvents. This classification is almost coterminous with that of 

 Hardy (1600, 1 and 2) into reversible and irreversible colloids, according to 

 whether, after evaporation to dryness, they go intq solution again on mere 

 addition of water or remain as a solid film. Typical instances of the hydrophile 

 class are gelatine and gum, of the hydrophobe class, gold and arsenious sulphide. 

 Intermediate forms are also known to exist, that is, systems which have some of 

 the properties of each class. Such are the sulphur preparations of Sven Ode'n 

 (1912, p. 712), which give reversible precipitates with salts, like the hydrophile 

 class, but are precipitated by very small concentrations of bivalent ions, like the 

 hydrophobe class. It must be admitted that the existence of these intermediate 

 kinds of colloidal systems deprives all classifications as yet proposed of much of 

 their theoretic value, although useful in practice. 



The names "sol" and "gel" introduced by Graham (1864, p. 321, p. 620 of 

 the Collected Edition, 1876) may be referred to here; a colloidal solution of silicic 

 acid, at first liquid, becomes gelatinous in process of time. The two states are 

 called "hydrosol" and "hydrogel" respectively, when the external phase is water. 

 When this is alcohol, " alcosol," and so forth. 



Some degree of confusion is apt to arise from the use of the words "homogeneous ' and 

 "heterogeneous" as applied to solutions. It is plain that no solution can be absolutely 

 homogeneous ; a molecule of water and one of sodium chloride cannot be in the same place at 

 the same time. Indeed, von Calcar and Lobry de Bruyn (1904, p. 218) thought that they had 

 succeeded in producing, by centrifugal force, changes of concentration in solutions of potassium 

 iodide. It is also clear that, if we make as our criterion of heterogeneity the power we 

 possess of separating the phases mechanically, as Bakhuis Roozeboom (1901, p. 9) does, 

 colloidal solutions cannot be called heterogeneous. The really important point is, following 

 the work of Willard Gibbs, whether the phenomena due to the possession of surfaces of . 

 contact, as shown by matter in mass, are also shown by the "particles" of the iiiternal phase 

 in colloidal solutions. About this there is no dispute ; but, to avoid misunderstanding, it is 

 perhaps advisable not to use the name " heterogeneous " in their case, and to speak of 

 colloidal solutions as " micro-heterogeneous," one or more of the phases being minutely 

 subdivided. 



Where then can we say that " molar " properties cease and " molecular " 

 properties begin ? The question remains as yet unanswered, but it seems clear 

 that a gradual transition must exist, and possibly some of the disputes as to the 

 relation between the chemical and physical properties of certain colloidal systems 

 may lie due to an exclusive consideration of a part only of the phenomena shown 

 by these intermediate states. 



Whatever phenomena are manifest at interfaces between phases will obviously 

 be greater as these interfaces increase in area. It is of interest, therefore, to 

 calculate the amount by which the surface of a given mass increases when sub- 

 divided to colloidal dimensions. The particles of gold in some of the preparations 

 of Siedentopf and Zsigmondy (1906) were found, by a method to be described 



