. The principles of chemistry . gelatin, or common carpenters glue, when soaked inwater, swells up into an insoluble jelly. If this jelly be heated, it melts, and is thensoluble in water, but on cooling it again forms a jelly which is insoluble in water. Oneof the properties which distinguish colloids from crystalloids is that the former pass veryslowly through a membrane, whilst the latter penetrate very rapidly. This may be shown• by taking a cylinder, open at both ends, and by covering its lower end with a bladder orwith vegetable parchment (unsized paper immersed for two or three minutes i
. The principles of chemistry . gelatin, or common carpenters glue, when soaked inwater, swells up into an insoluble jelly. If this jelly be heated, it melts, and is thensoluble in water, but on cooling it again forms a jelly which is insoluble in water. Oneof the properties which distinguish colloids from crystalloids is that the former pass veryslowly through a membrane, whilst the latter penetrate very rapidly. This may be shown• by taking a cylinder, open at both ends, and by covering its lower end with a bladder orwith vegetable parchment (unsized paper immersed for two or three minutes in a mixtureof sulphuric acid and half its volume of water, and then washed), or any other mem-branous 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 an apparatus is called a dialyser (fig. 15), and the process of separation of crystal-loids from colloids by means of such a membrane is termed dialysis. An aqueous. Fig. 15.—-Dialyser. Apparatus for the separation of sub-stinces wlii-h tlirough a membraue from thosewhich do not. Description in text. 64 PRIXCIPLES OF CHEMISTKY Hence, if it be desired to increase the rate of solution, recoursemust be had to stirring, shaking, or some such mechanical if once a uniform solution is formed, it will remain uniform, nomatter how heavy the dissolved substance is, or how long the solutionbe left at rest, which fact again shows the presence of a force holdingtooether the particles of the body dissolved and of the solvent.^ solution of a crystalloid or colloid, or a mixture of both, is poured into the dialj-ser,which is then placed in a vessel containing water, so that the bottom of the membraneis covered with water. Then, after a certain period of time, the crystalloid iDasses throughthe membrane, whilst the colloid, if it does pass through at all, does so at an incompar-ably slower rate. The crystalloid na
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