SEVENTEENTH AND EIGHTEENTH CENTURIES 165 

 some wonderful pioneering work. One of the chief results of his observations 

 was his discovery of oxygen, which he found by heating mercury monoxide; 

 further, he experimented successfully with carbonic acid, which brought 

 him into the sphere of vegetable and animal chemistry. He found that rats 

 kept in a volume of air that was confined by water died as a result of the 

 pollution of the air, but by letting green plants stand for a time in that 

 same air, it was so improved that fresh rats were again able to live in it 

 for some time. He found by a series of experiments that the air polluted by 

 the animals' breathing contains carbonic acid, or "fixed air," as he called 

 it. As a theorist Priestley was not particularly original; up to his death he 

 stubbornly maintained the phlogiston theory, which had already been aban- 

 doned by most chemists of his age. 



The scientist who put the chemistry of combustion on the right road 

 was Antoine Laurent Lavoisier. He was born in Paris in 1743, the son 

 of a lawyer, and was given an excellent education, special attention being 

 paid to mathematics and natural science. He went in for an official career, 

 however, and in time became a "farmer-general" — that is, a titular mem- 

 ber of a body to which the French Government had leased the collection 

 of revenue. This system naturally gave rise to a good deal of abuse, and its 

 officials were not much better tolerated than the publicans of the Jews of 

 old. Lavoisier had never been guilty of fraud, but when the Revolutionary 

 tribunal condemned his colleagues, he was likewise involved in their fall. 

 Condemned for no reason at all, he was guillotined by the Terrorists in 

 1794. When his services to science were cited as grounds for mercy, the peti- 

 tion was met with the reply: "La Republique n a pas besoin de savants." 



Lavoisier founds quantitative chemistry 

 It has been said of Lavoisier that he never discovered a new substance or 

 a new phenomenon, but that he introduced a new spirit into his science. 

 Even the system of weights and measures on which he based his reform had 

 been used before him by Hales and others, but Lavoisier was the first who, 

 in the study of chemical phenomena, consistently paid attention to the 

 weight conditions and in each chemical process determined their immuta- 

 bility, thereby making of chemistry an exact science in the same way as 

 physics. Thanks to Priestley's discovery of oxygen, he was able to account 

 for combustion and he gave the name of "oxgyen" to the gas which had 

 formerly been called " dephlogisticated air." Likewise, he established the 

 fact of water's being composed of oxygen and hydrogen, the latter discovered 

 by Cavendish. Moreover, he found out that heat is unweighable — a fact 

 which still further explained the process of combustion. He also applied his 

 weighing method to life-phenomena; he shut up animals in a confined volume 

 of air and by means of weighing determined the change brought about by 

 their breathing therein. He established the fact that oxygen is the component 



