JOSEPH PRIESTLEY. 115 



quently formulated by Lavoisier, that, in chemical pro- 

 cesses, matter is neither created nor destroyed, and in- 

 dicated the path along which all future explorers must 

 travel. Nor did he himself halt until this path led 

 him, in 1781, to the brilliant and fundamental discovery 

 that water is composed of two gases united in fixed 

 and constant proportions. 



It is a trying ordeal for any man to be compared 

 with Black and Cavendish, and Priestley cannot be said 

 to stand on their level. Nevertheless, his achievements 

 are not only great in themselves, but truly wonderful, 

 if we consider the disadvantages under which he la- 

 boured. Without the careful scientific training of Black, 

 without the leisure and appliances secured by the wealth 

 of Cavendish, he scaled the walls of science as so many 

 Englishmen have done before and since his day ; and 

 trusting to mother wit to supply the place of training, 

 and to ingenuity to create apparatus out of washing 

 tubs, he discovered more new gases than all his prede- 

 cessors put together had done. He laid the founda- 

 tions of gas analysis ; he discovered the complementary 

 actions of animal and vegetable life upon the constitu- 

 ents of the atmosphere ; and, finally, he crowned his work, 

 this day one hundred years ago, by the discovery of 

 that " pure dephlogisticated air " to which the French 

 chemists subsequently gave the name of oxygen. Its 

 importance, as the constituent of the atmosphere which 

 disappears in the processes of respiration and combus- 

 tion, and is restored by green plants growing in sun- 



