LECTURE X.] HISTORY OF CHEMISTRY. 177 



the whole is now a single coherent substance. We can picture 

 to ourselves ... the elements in the atom of the salt as 

 coupled together in various ways ; for example, as one atom of 

 sulphide of copper combined with four atoms of oxygen, that 

 is to say, as the oxide of a compound radical ; as one atom of 

 binoxide of copper and one atom of sulphurous acid ; as one 

 atom of copper and one atom of a salt-former, SO 4 ; and, 

 finally, as one atom of oxide of copper and one atom of sul- 

 phuric acid. So long as the simple atoms remain together, 

 one of these notions is as good as another. If it is a question, 

 however, of the behaviour when the compound atom is decom- 

 posed by electricity, or by the action of other substances (in 

 the wet way for instance), then the relation is quite different. 

 The compound atom in that case never undergoes decom- 

 position in accordance with the first two views, but it does 

 according to the two latter. The copper can be exchanged for 

 other metals according to the view Cu + SO 4 ; but if the copper 

 is taken away without being replaced, as is the case by the action 

 of electricity, then that part of the atom of the salt which 

 remains over, breaks up into oxygen and sulphuric acid. If, 

 on the contrary, the salt of copper is decomposed either by a 

 very feeble electrical force or by means of other oxides, into 

 oxide of copper and sulphuric acid, both of these remain after- 

 wards, and from them the salt can be compounded again. There 

 must naturally be a reason for these circumstances, and the reason 

 can scarcely be other than this, that when sulphuric acid and 

 oxide of copper unite to form a compound atom of the salt, the 

 relative positions of the atoms in the united binary substances 

 do not materially change, and the latter can thus be combined 

 or separated as often as is desired. . . . From this, however, 

 it easily follows that in decomposing to form other binary com- 

 pounds of the elements, the atoms must undergo a trans- 

 position of their relative situations, so that their capacity for 

 combining anew is either diminished, or, as is usual, ceases 

 entirely. Nitrate of ammonia, which is decomposed into nitric 

 acid, ammonia, and water, and is recompounded from these, 

 can be decomposed by heat into nitrous oxide and water, 



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