and Attached Water. 219 



following composition : 5*1304 grms. of solution gave 2*7233 

 grms. of C 4 H 6 4 , or 53*0 per cent. 



(3) Colloids in aqueous solution ; their behaviour on being 

 cooled and on being heated. 



§ 161. Generalities.— It is perhaps only by an extension of 

 the definition that we can regard solutions of colloids as solutions 

 of salts. It will be of great interest to examine how far the 

 two classes of bodies resemble, and how far they differ from 

 one another in aqueous solution. The substantial researches 

 of Graham on the division of matter into crystalloids and col- 

 loids, resulting as they did in the discovery of a whole series 

 of new and profoundly interesting bodies, threw a flood ot 

 light on molecular physics. He showed that colloid septa 

 were permeable only to crystalloid liquids, and thus established 

 among the molecular mechanism of matter a series of con- 

 nected facts similar to and of no less importance than those 

 which have of late attracted so much attention in the domain 

 of radiation ; for that sonorous undulations are absorbed by 

 sympathetic vibrators, that radiation is arrested by the possi- 

 ble authors of such radiation, cannot long be separated from the 

 fact that colloids arrest colloids by contact-continuity, while 

 they are permeable (transparent, diathermanous) to crystalloids. 



Recalling a few of the distinguishing properties of colloids, 

 we find that when miscible at all with water they are misci- 

 ble in all proportions. Accordingly one colloid cannot pre- 

 cipitate another from solution. Towards water they then 

 behave as two gases behave towards one another. gome 

 gelatinize (gelatine) ; others do not (gum-arabic). borne 

 coagulate by heat (albumen) in association with water. In 

 the dry state they are amorphous and show conchoidal l rac ~ 

 ture. I look upon a water-jelly as consisting of an indefinitely 

 minute cellulation of a solid colloid enclosing liquid water. 

 In a liquid water-colloid the solid colloid particles are present, 

 but they do not form closed cells, and therefore do not with 

 their contents resist change of form ; nor are they necessarily 

 in contact with one another. The viscosity of such a mass is 

 due to friction between its solid and liquid parts. The elas- 

 ticity of a jelly is due to the elasticity of its cell-walls. Thus 

 I can form a model of a jelly by pressing together little 

 caoutchouc bladders containing water : and in soap-suds I 

 recognize a jelly of a higher order ; that is, instead of solid 

 we have liquid cell-walls, and instead of liquid we have 

 gaseous contents. On heating or stirring the suds the bub- 

 bles burst, the liquid walls collapse and contract, not again to 

 be formed automatically. On heating or stirring a jell}'- the 



