14 THE VITAL FUNCTIONS. 



had previously ascertained 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 addi- 

 tional weight of one hundred and seventy 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-chestnut 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, 

 and evidently derived little or no sustenance from 

 the w^ater with which they were supplied. Experi- 

 ments of a similar nature were made by Bonnet, 

 and with the like result. When plants are con- 

 tained in closed vessels, and regularly supplied 

 with water, but denied all access to carbonic acid 

 gas, they are developed only to a very limited ex- 

 tent, determined by the store of nutritious matter 

 which had been already collected in each plant 

 when the experiment commenced, and which, by 

 combining with the water, may have afforded a 

 temporary supply of nourishment. 



