154 DIMORPHISM, 



beautiful example of this change. When heated it appears to 

 burn, emits light, and becomes yellow, but undergoes no 

 change in weight. Many bodies exhibit a feeble phosphores- 

 cence when heated, which has no relation to this change and 

 is to be distinguished from it. 



The circumstance most certain respecting this change in 

 bodies, which affects so deeply their chemical properties, is 

 that the bodies do not contain a quantity of heat, after the 

 change, which they must have possessed before its occurrence 

 in a combined or latent form. No ponderable constituent is 

 lost, but there is this loss of heat. A change of arrangement 

 of the particles, it is true, might occur at the same time in some 

 of these bodies, such as is observed when sulphite of soda is 

 converted by heat into a mixture of sulphate of soda and sul- 

 phuret of sodium, without change of weight ; but it would be 

 difficult to apply an explanation of this nature to oxides, such 

 as alumina and peroxide of tin, which contain only two con- 

 stituents. The loss of heat observed will afford all the ex- 

 planation necessary if heat be admitted as a constituent of 

 bodies equally essential as their ponderable elements. As the 

 oxide of chromium possesses more combined heat when in the 

 soluble than in the insoluble state, the first may justly be viewed 

 as the higher caloruret, and the body in question may have 

 different proportions of this as well as of any other constituent. 

 But it is to be regretted that our knowledge respecting heat as 

 a constituent of bodies is extremely limited ; the definite pro- 

 portion in which it enters into ice and other solids in melting, 

 and into steam and vapours has been studied, and also the pro- 

 portion emitted daring the combustion of many bodies, which 

 has likewise proved to be definite. But the influence which its 

 addition or subtraction may have on the chemical properties of 

 a body is at present entirely matter of conjecture. The pheno- 

 mena under consideration seem to require the admission of heat 

 as a true constituent which can modify the properties of bodies 

 very considerably, otherwise a great physical law must be 

 abandoned, namely, that " no change of properties can occur 

 without a change of composition /* But if heat be once ad- 

 mitted as a chemical constituent of bodies, then a solution 

 of the present difficulties may be looked for, for nothing is more 

 certain than that " a change in composition will account for any 

 change in properties." Heat thus combined in definite pro- 



