270 POPULAR SCIENCE MONTHLY. 



this may not be a sharp line of division, it is highly probable that 

 typical members of two classes exhibit these properties of true solutions 

 in such a different degree as to make the differentiation an important 

 one. Unfortunately, however, colloidal mixtures have not yet been 

 satisfactorily enough investigated with respect to these properties to 

 enable a classification to be based exclusively upon them. 



Colloidal Solutions. 



Let us now consider the characteristics of the two classes as mani- 

 fested by typical representatives, beginning with the colloidal solutions. 

 These substances are, for the most part, obtained directly from animal 

 or vegetable sources and are purified by dialysis. Among the most 

 important are gelatine, agar-agar, unheated albumen, caramel, starch, 

 dextrine, and many natural gums. 



A number of the important properties of these colloidal solutions 

 have already been alluded to, but some of them deserve further con- 

 sideration. The contrasts and similarities between them and ordinary 

 solutions may first be mentioned. Such colloids possess a much slower 

 rate of diffusion, a much smaller osmotic pressure, and a much slighter 

 influence on the vapor-pressure, freezing-point, and boiling-point of 

 the solvent than do corresponding weights of crystalline substances. 

 So small are these effects that whether they exist at all is a question 

 to which much investigation and discussion have been devoted. The 

 now existing experimental data seem to show, however, that the gel- 

 atinizing non-coagulable colloids do possess these properties and influ- 

 ences in an appreciable degree. The results of the osmotic pressure 

 determinations in the cases where it has been measured against a 

 parchment or animal membrane, which would not retain the mineral 

 impurities, are especially significant. Thus by this method it has been 

 found that a 6 per cent, glue solution exerts a pressure of about one 

 third of an atmosphere, and that a 10 per cent, solution of the colloids 

 of blood-serum produces one of 40 mm. of mercury. Further investi- 

 gations in this direction would be of great value. The results of 

 Graham, too, seem to leave no doubt as to the existence of diffusion; 

 he found, for example, that albumen diffused one seventh as fast, and 

 caramel one fourteenth as fast as cane-sugar. Thus, these colloids 

 exhibit the same properties as ordinary dissolved substances, but in a 

 lesser degree — a fact which is explained in accordance with the mod- 

 ern theory of solutions by the simple assumption that they are true 

 solutions, but that their molecules consist of aggregates of the ultimate 

 chemical molecules and are, therefore, of much greater weight and 

 complexity than those of non-colloidal substances. 



This assumption seems, however, of itself alone, scarcely sufficient to 

 account for the abnormal viscosity of these colloids, their power of 



