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 bad 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 



