207. The first correct experiments, made to as- 
certain the specific changes induced on atmospheric 
air by the growth of seeds, were published by the 
celebrated Scheele, in his treatise on Air and Fire, 
about the year 1*777. He found that the air, in 
which moistened peas were made to grow, under- 
went no change of volume ; but that its oxygen gas 
disappeared, and a quantity of carbonic acid was pro- 
duced. As the quantity of acid formed equalled in 
bulk that of the oxygen gas which the air naturally 
contained, he proceeded to ascertain whether this gas 
had not been converted into the acid in question. 
With this view, he mixed together, in a bottle, one 
part of oxygen with three parts of nitrogen gas, and 
put some peas into it, with water sufficient to carry 
on their germination. The peas soon began to grow, 
and when they grew no more, he observed no varia- 
tion iu the bulk of air, but one-fourth of this air was 
attracted by milk of lime. It is therefore, says he, 
the oxygen gas of the air which, by germination, is 
changed into carbonic acid. This conclusion he far- 
ther established by confining peas in pure oxygen 
gas, where, though they did not grow so well, yet 
they changed it into carbonic acid ; and had he con- 
tinued the experiment a sufficient time, he had no 
doubt but that the whole of this gas would have ex- 
perienced a similar change *. 
208. Dr Ingenhousz confirmed and extended these 
facts, by proving that every kind of air, which was 
fatal to animal life, was incapable of supporting ger- 
* On Air and Fire, p, 151. 
