16 THK VITAL FUNCTIONS. 



modern chemistry it was very generally believed 

 that plants could subsist on vrater alone ; and 

 Boyle and Van Helmont in particular endea- 

 voured to establish by experiment the truth of 

 this opinion. The latter of these physiologists 

 planted a willow in a certain quantity of earth, 

 the weight of which he had previously ascer- 

 tained with great care ; and during five years, he 

 kept it moistened with rain water alone, which 

 he imagined was perfectly pure. At the end of 

 this period he found that the earth had scarcely 

 diminished in weight, while the willow had 

 grown into a tree, and had acquired an ad- 

 ditional weight of one hundred and fifty pounds : 

 whence he concluded that the water had been 

 the only source of its nourishment. But it does 

 not seem to have been at that time known that 

 rain water always contains atmospheric air, and 

 frequently also other substances, and that it 

 cannot, therefore, be regarded as absolutely pure 

 water : nor does it appear that any precautions 

 were taken to ascertain that the water actually 

 employed was wholly free from foreign matter, 

 which it is easy to conceive it might have held 

 in solution. In an experiment of Duhamel, on 

 the other hand, a horse-chesnut tree and an oak, 

 exposed to the open air, and watered with 

 distilled water alone, the former for three, and 

 the latter for eight years, were kept alive, indeed, 

 but they were exceedingly stinted in their growth, 



