20 Mac Leod, The Place of Science in Histoiy. 



Watt gave to the steam engine its modern form, an 

 ancient science, chemistry, which until then had been 

 hesitating and wrapped in mystery, suffered a profound 

 change, and became an exact science. Amongst the 

 scholars who have contributed to bring about this trans- 

 formation it is only fitting to name Lavoisier in the first 

 rank. 



Antoine Laurent Lavoisier was born at Paris, 

 on August 26th, 1743, and was executed on the Place de 

 la Revolution on May 8th, 1794. By his general use of 

 the balance for the measurement of chemical phenomena, 

 Lavoisier infused a new spirit into the science of chemistry. 

 In other words he applied the quantitative method to 

 chemistry. He set out from the principle that in every 

 chemical reaction the total weight of all the substances 

 produced is necessarily equal to the total weight of all 

 the substances employed. He supposed, besides, that the 

 heat generated by chemical reactions is without weight, 

 and thus avoided the error made by many of his pre- 

 decessors. Lavoisier's discoveries are so numerous, that 

 we must limit ourselves to noting some of the more 

 important. He laid down that sulphur and phosphorus 

 increased in weight through burning, by absorption of 

 air. After the preparation of oxygen (dephlogisticated 

 air) by PRIESTLEY in 1774, and after having laid down 

 that in these combustions only a part of the air is 

 absorbed, he concluded that atmospheric air is composed of 

 two substances, the new air (dephlogisticated air) of 

 Priestley, which is absorbed by phosphorus, sulphur, etc., 

 and non-vital air (azote) which plays no part in the com- 

 bustion. In a paper which appeared in 1782 he called 

 dephlogisticated air oxygen, that is, producer of acid, and 

 he formulated the hypothesis that acids are produced by 

 the union of oxygen with some simple body, ordinarily 



