173 
Dr. Thompson pursues the preceding subject as follows, 
shewing the dependence of vegetables on water, air, and soil, viz. 
177 Water indispensable to Vegetation. “In the first place 
it is certain that plants will not vegetate without water; for 
whenever they are deprived of it they wither and die. Hence 
the well-known use of rains and dews, and the artificial water- 
ing of ground. Water, then, is at least an essential part of the 
food of plants. But many plants grow in pure water; and 
therefore it may be questioned whether water is not the only 
food of plants. This opinion was adopted very long ago, and 
numerous experiments have been made in order to demonstrate 
it. Indeed it was the general belief of the seventeenth century; 
and some of the most successful improvers of the physiology of 
plants, in the eighteenth century, have embraced it. The most 
zealous advocates for it were. Van Helmont, Boyle, Bonnet, 
Duhamel, and Tillet. 
Van Helmont planted a willow which weighed five pounds in 
an earthen vessel filled with ’iOO lbs. of soil, previously dried in 
an oven, and then moistened with rain water. This vessel he 
sank into the earth, and he watered his willow, sometimes with 
rain and sometimes with distilled water. After five years it 
weighed 169} lbs. and the eartli in which it was planted, when 
again dried, was found to have lost only two ounces of its original 
weight. Here it has been said, was an increase of 164 lbs. and 
yet the only food of the willow was jiure water; therefore it Ibllows 
that pure w ater is sufficient to afford nourishment to plants. The 
insufficiency of this experiment to decide the question was first 
pointed out by Bergmann, in 1773. He showed, from the ex- 
periments of Margraft’, that the rain water employed by Van Hel- 
mont contained in it as much earth as could exist in the willow 
at the end of five years. For according to the experiments of 
Margraff, one pound of rain w ater contains one grain of earth. 
'I’he growth of the willow, therefore, by no means proves that 
the earth which plants contain has been formed out of water. 
Besides as 3Ir. Kirwan has remarked, the earthen vessel must 
have often absorbeil moisture from the surrounding earth, im- 
pregnated with whatever substance the earth contained; for 
unglazed earthen vessels, as Hales and Tillet have shown, 
readily transmit moisture. Hence, it is evident that no conclu- 
sion w hatever can be drawn from this experiment ; for all the 
187 aUCTABICM. 
