290 PHYSICAL CHANGES. 



neither : they appear to have destroyed the distinguishing 

 characters of each other. Combined bodies frequently 

 occupy less space than they did previously to combination, 

 of which numerous particular instances might be adduced. 

 Gases in many cases undergo a remarkable condensation 

 when chemically combined. In slaking lime, the water 

 becomes solid in the molecules of the hydrate of lime 

 formed, and the intense heat produced arises from the 

 liberation of that caloric which had been employed to 

 keep the water liquid. When a solid passes into the 

 liquid state, cold is produced by the abstraction from 

 surrounding objects of the heat required to effect fluidity. 

 An alteration of temperature occurs whenever chemical 

 change takes place, as we have already shown, with a 

 few trivial and uncertain exceptions. The disturbance 

 caused by the exercise of the force of affinity frequently 

 leads to the development of several physical powers. 



Changes of colour commonly arise ; indeed, there 

 does not appear to be any relation between the colour of 

 a compound and that of its elements. Iodine is of a 

 deep iron-grey colour ; its vapour is violet ; yet it forms 

 beautifully white salts with the alkalies, a splendid red 

 salt with mercury, and a yellow one with lead. The 

 salts of iron vary from white and yellow to green and 

 dark brown. Those of copper, a red metal, are of a 

 beautiful blue and green colour, and the anhydrous 

 sulphate is white. 



Isomorphism, which appears in a very remarkable 

 manner among the organic compounds, has, under the 

 head of crystallization, already had our attention. There 

 is also a class of bodies which are said to be isomeric ; 

 that is, to have the same composition, although different 

 in their physical characters. But the idea that bodies 

 exist, which, although of a decidedly different external 

 character, are of exactly the same chemical composition 

 and physical condition, is not tenable ; and in nearly all 

 the examples which have been carefully examined, a 



