ON WATKR AM ITS COMPOUNDS M 



;i nnMiibrane slowly, and form jellies ; that is, occur in insoluble 

 forms. 18 



18 The rate of diffusion like the rateof transmission through membranes, or tlitih/*/* 

 (which plays an important part in the vital processes of organisms and also in technical 

 work). present*, according to the researches of Graham, a sharply-defined change in 

 passing from such crystallisable substances as the majority of salts and acids to sub- 

 stances which are capable of giving jellies (gum, gelatin, il'c.). The former diffuse into 

 solutions and pass through membranes much more rapidly than the latter, and Graham 

 therefore distinguishes between cri/xtdlloid.i, which diffuse rapidly, and colloids, which 

 diffuse slowly. On breaking solid colloids into pieces, a total absence of cleavage is 

 remarked. The fracture of such substances is like that of glue or glass. It is termed a 

 conchoidal ' fracture. Almost all the substances of which animal and vegetable bodies 

 consist are colloids, and this is, at all events, partly the reason why animals and plants 

 have such varied forms, which have no resemblance to the crystalline forms of the 

 majority of mineral substances. The colloid solid substances in organisms that is, in 

 animals and plants are usually soaked with water, and take most peculiar forms, of net- 

 works, of grannies, of hairs, of mucous, shapeless masses, Arc., which are quite different 

 from the forms taken by crystalline substances. When colloids separate out from solu- 

 tions, or from a molten state, they present a form which is similar to that of the liquid 

 from which they were formed. Glass maybe taken as the best example of this. Colloids 

 are distinguishable from crystalloids, not only by the absence of crystalline form, but by 

 many other properties which admit of clearly distinguishing both these classes of solids, 

 as was shown by the above-mentioned English scientific man, Graham. Nearly all 

 colloids are capable of passing, under certain circumstances, from a soluble into an 

 insoluble state. The best example is shown by white of eggs (albumin) in the raw and 

 soluble form, and in the hard-boiled 

 and insoluble form. The majority 

 of colloids, on passing into an in- 

 soluble form in the presence of 

 water, give substances having a 

 gelatinous appearance, which is 

 familiar to every one in starch, 

 solidified glue, jelly, itc. Thus 



gelatin, or common carpenter's ^^^^f 



glue, when soaked in water, swells 

 up into an insoluble jelly. If this 

 jelly be heated, it melts, and is then 

 soluble in water, but on cooling it 

 again forms a jelly which is in- 

 soluble in water. One of the pro- 

 perties which distinguish, colloids Fl(i - 15.- Dialyser. Apparatus for tie separation of sub- 

 , . , . ,, , ,, , stances winch jwss through a membrane from those 



from crystalloids is that the former which do nor. Description in text. 

 pass very slowly through a mem- 

 brane, whilst the latter penetrate very rapidly. This maybe shown by taking a cylinder, 

 open at both ends, and by covering its lower end with a bladder or with vegetable parch- 

 ment (unsized paper immersed for two or three minutes in a mixture of sulphuric acid and 

 half its volume of water, and then washed), or any other membranous substance (all such 

 substances are themselves colloids in an insoluble form). The membrane must be firmly 

 tied to the cylinder, so as not to leave any opening. Such an apparatus is called a 

 fliuli/ser (fig. 15), and the process of separation of crystalloids from colloids by means of 

 such a membrane is termed dialysis. An aqueous solution of a crystalloid or colloid, 

 or a mixture of both, is poured into the dialyser, which is then placed in a vessel con- 

 taining water, so that the bottom of the membrane is covered with water. Then, after a 

 certain period of time, the crystalloid passes through the membrane, whilst the colloid, 

 if it does pass through at all, does so at an incomparably slower rate. The crystalloid 



