38 LESSONS IN CHEMISTRY. 



exercise a considerable influence upon its use in various opera- 

 tions, and especially upon the animals which use it for drink. 

 Boussingault, the celebrated agricultural chemist, has du-ected 

 his attention to this subject, and has calculated that by the salts 

 dissolved or held in solution in the water used as drink by 

 his cattle, 2 cwt. of alkaline salts (13) were added to his 

 dung heap every year. 



36. Water, therefore, by its power of absorbing the gases 

 of the air, and dissolving the mineral matter of the soil, affords 

 a means by which these substances may be introduced into 

 the interior of the plant, and we shall see that it is requisite 

 that all the matters which the soil supplies should be dis- 

 solved in this useful fluid before they can contribute to 

 promote vegetation. 



37. As in the laboratory of the chemist water can be decom- 

 posed into its elements, so when it penetrates into the interior 

 of the plant, it can undergo decomposition under the influence 

 of those curious agencies which the living vegetable is capa- 

 ble of exercising upon matter, its hydrogen being employed 

 in the production of various compounds. Water is regarded 

 as the chief source of the hydrogen of plants ; and when it is 

 worked up in the vegetable structure, oxygen is separated in 

 its gaseous form, and returned to the atmosphere. 



38. The quantity of water which is annually deposited 

 upon the earth in rain differs very much in different coun- 

 tries. Over the whole earth it is estimated at from 32 to 33 

 inches, but from various causes the proportion of moisture 

 which some countries receive greatly exceeds this amount. 

 Thus, in different parts of England (Dr. Prout), it varies 

 from 22 inches, as at London, to 68 inches, at Keswick, 

 whilst at St. Domingo it amounts to so much as 150 inches. 

 The water of the ocean is constantly passing into vapour 

 and forming clouds, which are conveyed by the winds to a 

 considerable distance into the interior of a country. From 

 the peculiar position of Ireland, with its west coast exposed 

 to the currents which carry with them clouds charged with 

 the moisture exhaled from the immense expanse of the At- 

 lantic, and which from various causes, and especially from 

 coming into contact with the cold mountain ranges that fringe 

 our coasts, have their temperature so much reduced that they 

 are condensed into rain, which is precipitated in frequent 

 showers over the land, the moisture of the climate of this coun- 



