A HISTORY OF SCIENCE 



of expense, which, thanks to his inheritance, he could 

 ignore. In this process he found that a gas was given 

 off which precipitated lime from water, and proved to 

 be carbonic acid. Observing this, and experimenting 

 with other substances known to give off carbonic acid 

 in the same manner, he was evidently impressed with 

 the now well-known fact that diamond and charcoal 

 are chemically the same. But if he did really believe 

 it, he was cautious in expressing his belief fully. " We 

 should never have expected," he says, " to find any re- 

 lation between charcoal and diamond, and it would be 

 unreasonable to push this analogy too far ; it only exists 

 because both substances seem to be properly ranged in 

 the class of combustible bodies, and because they are 

 of all these bodies the most fixed when kept from con- 

 tact with air." 



As we have seen, Priestley, in 1774, had discovered 

 oxygen, or " dephlogisticated air." Four years later 

 Lavoisier first advanced his theory that this element 

 discovered by Priestley was the universal acidifying or 

 oxygenating principle, which, when combined with char- 

 coal or carbon, formed carbonic acid ; when combined 

 with sulphur, formed sulphuric (or vitriolic) acid ; with 

 nitrogen, formed nitric acid, etc., and when combined 

 with the metals formed oxides, or calcides. Further- 

 more, he postulated the theory that combustion was 

 not due to any such illusive thing as "phlogiston," 

 since this did not exist, and it seemed to him that the 

 phenomena of combustion heretofore attributed to 

 phlogiston could be explained by the action of the new 

 element oxygen and heat. This was the final blow to 

 the phlogiston theory, which, although it had been tot- 



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