334 THE LIFE OF THE PLANT 



as it appears to us now ; hence we are the more amazed at the 

 brilliant deduction, at the ingenious conception to which the 

 world owes one of its greatest discoveries in biology. Priestley 

 proved by a series of experiments that continual combustion, 

 or continual respiration in a limited volume of air, makes that 

 air unfit for further combustion, for further respiration : in it 

 a lighted candle goes out, an animal dies. Therefore, argued 

 Priestley, all the atmosphere should become unfit for combus- 

 tion, or for life ; yet the many centuries of the world's existence 

 testify to the contrary. Apparently there exists a process in 

 Nature which restores this had air into good air. Is this not 

 due to plants ? On the i8th of August 1772 Priestley made 

 the following experiment. He introduced under a glass bell 

 over water, where a candle had previously gone out, or a 

 mouse had died, a plant (mint), and kept it there for a time. 

 The plant did not perish ; it even continued to develop, and 

 when after a few days a mouse, or a burning candle, was again 

 introduced under the glass bell, it appeared that the air had 

 actually been renewed, that it had acquired once more the proper- 

 ties of maintaining combustion and respiration. Hardly ever in any 

 province of knowledge has a single experiment been followed by 

 greater results. The same stroke demonstrated the most charac- 

 teristic sides of the life of plants and animals, and the mutual 

 relationship which exists between the two kingdoms of Nature. 

 Priestley's contemporaries appreciated the importance of this dis- 

 covery. The Royal Society conferred on him the coveted Copley 

 medal ; and the President of the Society, Sir John Pringle, ex- 

 pressed the importance of Priestley's achievement in the following 

 eloquent, though somewhat rhetorical, words : ' From this dis- 

 covery,' says he, ' we are assured that no vegetable grows in vain, 

 but that, from the oak of the forest to the grass in the field, every 

 individual plant is serviceable to mankind ; if not always distin- 

 guished by some private virtue, yet making a part of the whole, 

 which cleanses and purifies our atmosphere. In this the fragrant 

 rose and deadly nightshade co-operate : nor is the herbage, 

 nor the woods that flourish in the most remote and unpeopled 

 regions, unprofitable to us, nor we to them ; considering how 

 constantly the winds convey to them our vitiated air for our 

 relief and for their nourishment.' Priestley's inference was that 

 the plant restored air vitiated by respiration, and made it again 



