io8 DIALYSIS. PARTI. 



hot, lukewarm, or cold. Thus each particle of water, 

 containing one atom of oxygen and one of hydrogen, 

 combines with one atom of zinc in three different pro- 

 portions as if it were a simple atom. 



The water of crystallization may be driven off from 

 many substances by heat, as from the hydrates of lime, 

 iron, copper, &c., but when combined with the oxides of 

 certain metals, potassium for instance, it cannot be 

 driven off by any means whatever. In general a heat 

 of 212 Fahr. is sufficient, but some crystals lose their 

 water of crystallization at the ordinary atmospheric 

 temperature. 



Crystals whose atoms are in unstable equilibrium, are 

 readily altered both externally and internally by a very- 

 moderate degree of heat. Arragonite and calcareous 

 spar are isonieric, that is, they are chemically the same 

 but differ in form and hardness, which shows that their 

 molecules are grouped differently. When the arragonite 

 is heated, the inertia of its atoms is overcome, the crystal 

 explodes with force, and becomes a mass of crystals of 

 calcareous spar. The expansive force of the heat sud- 

 denly overcoming the force of cohesion causes the ex- 

 plosion, and at the same time disturbs the unstable 

 repose of the atoms, which immediately obey their 

 natural attractions and assume the stable form of cal- 

 careous spar. 



Dialysis is a method of separating and analysing 

 substances by means of their diffusion in alcohol or 

 water. If a wide-mouthed vial nearly full of a solution 

 of common salt be placed in a jar of water, after a few 

 days it will be found that the particles of salt have come 

 out of the vial and have diffused themselves through the 

 superincumbent water, even to its surface. Now Pro- 

 fessor Graham, Master of the Mint, with whom this 

 subject originated, made three arrangements precisely 

 like that described ; the three vials were exactly similar 



