ISOMERISM. 155 



portions with bodies and viewed as a constituent, must not be 

 confounded with the specific heat of the same bodies or their 

 capacity for sensible heat,, which may have no relation to their 

 combined heat. 



ISOMERISM. 



In such changes of properties the individuality of the body is 

 never lost. But numerous instances have presented themselves 

 of two or more bodies possessing the same composition, which 

 are unquestionably different substances and not mutually con- 

 vertible into each other. Different bodies thus agreeing in 

 composition but differing in properties are said to be isomeric, 

 (from icroQy equal, and juepoc, part), and their relation is 

 termed isomerism. The discovery of such bodies excited much 

 interest and they have received of late years a considerable 

 share of the attention of chemists. But the result of a careful 

 study of the bodies associated by similarity of form, though 

 differing in properties, has been upon the whole unfavourable 

 to the doctrine of isomerism. Isomeric bodies have in general 

 been found to agree in the relative proportion of their consti- 

 tuents only, and to differ either in the aggregate number of 

 the atoms composing them, or in the mode of arrangement of 

 these atoms ; and very few cases of isomerism now remain which 

 do not admit of explanation. This is what was to be expected, 

 for isomerism in the abstract is improbable, a difference in pro- 

 perties between bodies, without a difference in their composi- 

 tion, appearing to be an effect without a sufficient cause. Hence, 

 the term isomerism is now generally employed in a limited 

 sense., to indicate simply the identity in composition of two or 

 more bodies as expressed in the proportion of their constituents 

 in 100 parts. Several classes of such isomeric bodies may be 

 formed. 



The members of the most numerous class of isomeric bodies differ 

 in atomic weight. Thus we know at present three gases, three or 

 four liquids, and as many solids which all consist exactly of carbon 

 and hydrogen in the proportion of one atom to one atom, or in 

 weight, of 86 parts of carbon and 14 of hydrogen very nearly. 

 These bodies agree in ultimate composition, but differ com- 

 pletely in every other respect. But a representation of their 

 chemical constitution explains at once the cause of the diffe- 



