C 185 ] 



ed 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 ef- 

 fects will be produced upon the potassium, as if it had 

 absorbed a small quantity of oxygene ; and the hydro- 

 gene disengaged, and the oxygene added to the potas- 

 sium are in weight as 2 to 15 ; and if two in volume of 

 hydrogene, and one in volume of oxygene, which have 

 the weights of 2 and 15, be introduced into a close ves- 

 sel, and an electrical spark passed through them, they 

 will inflame and condense into 1 7 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 ele- 

 ments, enters largely into the constitution of their or- 

 gans and solid productions. 



Water is absolutely necessary to the ceconomy 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 be- 

 neath are protected by the congealed water from the 

 influence of the atmosphere, the temperature of which 



B2 



