84. SCIENTIFIC APPARATUS. 



be referred the absorption of oxygen by metallic silver, although 

 while occluded hydrogen is .expelled by heat, occluded oxygen 

 escapes as the metal cools, or at least as it solidifies. The forma- 

 tion of ammonium amalgam can scarcely be classed with the 

 above, because the volume of the metal is so notably increased, 

 and its metallic aspect, when saturated with the hydrogen-ammonia, 

 is so impaired. 



The passage of a solid into a liquid is due to the overcoming of 

 the cohesion of the solid, in the first instance, by the adhesion of 

 the liquid to it, and afterwards by that of the solution. While all 

 gases or vapours intermix with one another in all proportions, the 

 mixture between liquids and liquids sometimes takes place in all 

 proportions, and is sometimes limited in quantity, so that any less, 

 but no greater quantity than n of liquid A will dissolve in m of B. 

 And the same limitation always holds when solids dissolve in 

 liquids. This ratio of solubility cannot at present be deduced from 

 the elementary composition of the substances, but it may be broadly 

 laid down that bodies the most similar in elementary composition 

 mix the most abundantly. A liquid rich in carbon will, for instance, 

 dissolve a great many solids containing that element in combina- 

 tion. Water dissolves most abundantly those alcohols which stand 

 nearest to it in composition. The solubility of a solid in a liquid 

 is always increased by heat, except where the heat determines the 

 formation of some new and less soluble body. 



The solubility of a crystalline salt in water extends below trie 

 freezing-point of water, and accordingly, as a solution of a salt 

 saturated above the freezing-point is cooled, the salt or some 

 hydrate separates in larger and larger quantity until a certain tem- 

 perature below the freezing-point of water is reached, when the 

 remainder of the salt and the water solidify together at a constant 

 temperature and in a fixed ratio. The temperature is that which 

 results from the use of the same salt with ice as a freezing mixture, 

 and the ratio between the salt and the water in the solid formed 

 is the same as in the liquid part of such a freezing mixture. On 



