198 HISTORY OF CHEMISTRY. [LECTURE XI. 



deserves to be stated here. They observed that on burning 

 carbon in oxygen, less heat is produced than when nitrous oxide 

 is employed. They considered that this striking fact could 

 only be explained on the hypothesis that in both cases, besides 

 the formation of carbonic anhydride, a decomposition took 

 place ; that is to say, a change occurred involving the separa- 

 tion of atoms which had previously been combined. Accord- 

 ingly they thought that even in the free particles of oxygen gas, 

 several (two) atoms must be assumed, and that the quantity of 

 heat required for their decomposition must be greater than that 

 required for the separation of the oxygen from the nitrogen in 

 nitrous oxide. 



By the application of a hypothesis relating to chemical com- 

 bination and decomposition, Brodie 2 arrives at the divisibility 

 of molecules of hydrogen and of oxygen. It appears to him that 

 the contrast which was drawn, in accordance with the views 

 prevailing at that time, between the formation of compounds 

 and the separation of elements, has no natural foundation. In 

 his view, every combination is merely the consequence of a 

 decomposition, and this can only be occasioned by new com- 

 binations. He tries to .prove the accuracy of this view by 

 means of various examples, and, in doing so, introduces certain 

 signs and expressions designed to give an idea of the contrast 

 (more generally, of the relation) between the atoms entering 

 into combination. According to Brodie, there exists between 

 these a relation (polarity) of such a kind that the one is desig 

 nated as positive or negative as contrasted with the other. This 

 relation, which Brodie also calls chemical difference, depends 

 upon the peculiarities of all those particles with which the atom 

 is for the time being combined. 



To permit of a better comprehension of this, I give here some 

 of the examples adduced by Brodie. Silver does not unite 

 directly with oxygen, whereas silver chloride is decomposed by 

 boiling with potash, silver oxide being formed. According to 

 Brodie, this is due to the fact that it is only by their combina- 



2 Phil. Trans. 1850, 759. 



