ON THE CONSTITUTION OF THE ATMOSPHERE. 31 



.These facts are of great importance in the theciry and in the enlightened 

 practice of agriculture. They will hereafter come under special and 

 detailed consideration, when we shail have examined the nature of the 

 soils in which plants grow, and shall be prepared to consider the chemi- 

 cal nature, the source, and the functions, of the inorganic compounds 

 which exist in living animal and vegetable substances. 



§ 3. Of the form or state of combination in which the organic elements 

 enter into and minister to the growth of plants. 



From ihe details already presented in the preceding Lecture, in re- ^ 

 gard to the properties of carbon and tit<<rogen, and the circumstances M^ 

 under which they are met with in nature, — it will readily occur to you C' 

 that neither of these elementary bodies is likely to enter directly, or in a 

 simple state, into the circulation of plants. The former (carbon) being 

 a solid substance, and insoluble in water, cannot obtain admission into 

 the pores of the roots, the only parts of the plants with which, in nature, 

 it can come in contact. The latter (hydrogen) does not occur either in 

 the atmosphere or in«the soil in any appreciable quantity, and hence, in 

 its simple state, forms no part of the food of plants. Oxygen and nitro- 

 gen, again, both exist in the atmosphere in the gaseous state, and the 

 former is known to be inhaled, under certain conditions, by the leaves 

 of plants. Nitrogen may also in like manner be absorbed by the leaves 

 of living plants, but, if so, it is in a quantity so small as to have hitherto 

 escaped detection. The two latter substances (oxygen and nitrogen) 

 are also slightly soluble in water, and, besides being inhaled by the 

 leaves, may occasionally be absorbed in minute quantity along with the 

 water taken in by the roots. But by far the largest proportion of these 

 two elementary bodies, and the whole of the carbon and hydrogen • 

 which find their way into the interior of plants, have previously entered 

 into a state of mutual combination — forming what are called distinct 

 chemical compounds. Before describing the nature and constitution of 

 these compounds, it will be proper to explain, 1°. the constitution of the 

 atmosphere in which plants live, and, 2°. the nature of chemical com- 

 bination and the laws by which it is regulated. 



§ 4. On the constitution of the atmosphere. 



The air we breathe, and in which plants live, is composed principal- 

 ly of a mixture of oxygen and nitrogen gases, in the proportion very 

 nearly of 21 of the former to 79 of the latter. It contains, however, as 

 a constituent necessary to the very existence of vegetable life, a small 

 per centage of carbonic acid. On an average this carbonic acid 

 amounts to about a^^^th part* of the bulk of the air. On the shores 

 of the sea, or of great lakes, this quantity diminishes ; and it becomes 

 sensibly less as we recede from the land. It is also less by day than 

 by night (as 3*38 to 4*32), and over a moist than over a dry soil. 



The air is alsO imbued with moisture. Watery vapour is every 

 where diffused through it, but the quantity varies with the season of 

 the year, with the climate, with the nature of the locality, with its alti- 



• 0-04 per cent. The mean of IM experiments made by Saussure at Geneva at all times 

 of the year and of the day gave 4- .5 volumes in 10000. The maximum was 574, and th« 

 minimum 3 15. 



