AGRICULTURAL TEXT-BOOK. 13 



of magnesium contained in the waters of the lake of Haarlem, 

 by the action of the sun s raya, 



38. The quantity of water in tho atmosphere varies considei^- 

 ably, at different times and in different places. It is, besides, 

 dependent on the temperature of the air and of tho water evap 

 orating from the earth s surface. The proportion of aqueous 

 rspor has been determined by Verver, for the Netherlands. In 

 1000 volumes of air, he found the minimum 5.8 ; the maximum 

 10.18. The average of fifty observations during May, August, 

 and September was 8.47. From an early hour in the morning 

 to- 1.0 o clock, A. M. it was 7.97 from 10 to 2 o clock, P. M., 

 8.S8 ; and from 2 o clock till the evening, 8.85 (Mulder.) 



The atmosphere in New England is believed to be much 

 drier than that of Germany, At a meeting of the Boston Nat 

 ural History Society, 1852, this difference was discussed. In 

 New England the dew point is several degrees lower. Various 

 differences are noticed by foreign artizans in the processes of 

 their different callings, attributable to this dryness of the aii. 

 The climate of New England is regarded as more nearly resenv 

 IJing that of the high Alps than that of any other part of En- 

 rope. Missouri, on the contrary, appears to have a still wetter 

 atmosphera At the winter season of the year the air of New 

 England is so nearly anhydrous, (free from moisture y ) that such 

 articles as raw hides dry in a temperature just below freezing, 

 without being frozen. (Annual of Scitnt. Discov^ 1853 J 



39. All these constituents of the atmosphere have an impor 

 tant influence upon the character and luxuriance of vegetation* 

 Of ihe whole, however, carbonic acid and water are the most 

 important. Carbon, and the elements of water form the prin 

 ciple constituents of vegetables; the quantity of the substances 

 which do not possess this compositon being in a very small pro 

 portion, 



40. The leaves and other green parts of a plant exposed te&amp;gt; 



