Biographical Memoir ofDr Priestley. 215 



Priestley found this means in the property which he discover- 

 ed in vegetables of purifying the atmospheric air during the day, 

 by decomposing the fixed air, — a property M'hich is moreover the 

 principal key to the whole vegetable economy, and which, join- 

 ed to the property that animals have of vitiating the air by re- 

 spiring it, led to the fact, which has been subsequently more 

 clearly developed, that the spring of life principally consists in 

 a perpetual transformation of elastic fluids. 



Thus, his discoveries respecting the gases opened quite a 

 new field to the inquiries into living bodies : physiology and 

 medicine were enlightened from a source hitherto unknown. 

 New rays, still brighter, presently issued from the same focus. 

 Having applied the heat of a burning-glass to salts of mercury, 

 Priestley had the good fortune to obtain, pure and isolated, that 

 respirable portion of the atmospheric air which animals consume, 

 which vegetables restore, and combustion alters. He named it 

 dephlogisticated air. The other gases different from common air, 

 extinguished lights ; this made them burn with a clear flame, and 

 with prodigious rapidity : the others destroyed animals immersed 

 in them ; in this they lived even longer than in common air, 

 without requiring its renewal ; their faculties seemed to acquire 

 more energy in it. It was for a moment imagined that this dis- 

 covery afforded a new means of exciting and perhaps of prolong- 

 ing life, or at least an infallible remedy against most of the dis- 

 eases of the lungs. This hope was fallacious. Nevertheless the 

 dephlogisticated air remains one of the most brilliant discoveries 

 of the eighteenth century ; it is the same which, under the name 

 of oxygen, the modern chemist regards as the most universal 

 agent of nature. By it are produced combustion and calcina- 

 tion of every kind ; it enters into the composition of most of the 

 acids ; it is One of the elements of water, and the grand reser- 

 voir of fire ; it is to it that we owe almost all the artificial co- 

 lours which we make use of in common life, and in the arts ; it is 

 that which, in respiration, gives to our bodies, ^as well as to those 

 of animals, their natural heat, and the material principle of their 

 motions ; the energy of the various species of animals is in pro- 

 portion to the power of its action upon them ; vegetables pass 

 through no period of their growth, without its being combined 

 or disengaged in them in various ways: — in a word, natural 



