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economy of nature, that aqueous vapour is most 
abundant in the atmosphere when it is most needed 
for the purposes of life ; and that when other sources 
of its supply are cut off, this is most copious. 
The compound nature of water has been referred 
to. It may be proper to mention the experimental 
proofs of its decomposition into, and composition 
from oxygene and hydrogene. 
If the metal called potassium be exposed in a 
glass tube to a small quantity of water, it will act 
upon it with great violence ; elastic fluid will be 
disengaged, which will be found to be hydrogene ; 
and the same effects will be produced upon the 
potassium, as if it had absorbed a small quantity of 
oxygene ; and the hydrogene disengaged, and the 
oxygene added to the potassium, are in weight as 
c 2 to 15; and if two in volume of hydrogene, and 
one in volume of oxygene, which have the weights 
of c l and 15, be introduced into a close vessel, and 
an electrical spark passed through them, they will 
inflame and condense into 17 parts of pure water. 
It is evident from the statements given in the 
third Lecture, that water forms by far the greatest 
part of the sap of plants : and that this substance, 
or its elements, enters largely into the constitution 
of their organs and solid productions. 
Water is absolutely necessary to the economy 
of vegetation in its elastic and fluid state ; and it 
is not devoid of use even in its solid form. Snow 
and ice are bad conductors of heat ; and when the 
ground is covered with snow, or the surface of the 
soil or of water is frozen, the roots or bulbs of the 
plants beneath are protected by the congealed 
