288 CHEMISTRY IN AGRICULTURE AND PHYSIOLOGY. 
oxygen in twent} T -four hours; ten cwt. of charcoal consumes 
32.066 cubic feet of oxygen during its combustion (Liebig); 
so that a city like Utica, with about 21.000 inhabitants, ex¬ 
tracts yearly from the air, by wood and coal employed as 
fuel, more than ool millions of cubic yards, of this gas. It has 
been established by careful experiments, that the composition 
of the atmosphere is invariable : 100 volumes of air have been 
found in every period, and in every climate, to contain twenty- 
one volumes of oxygen to seventy-nine of nitrogen. 
A general notion of the manner in which the permanent 
character of the atmosphere is established may be gained from 
the following experiments. 
The leaves of plants are their exhaling and respiratory 
organs. 
For illustrating the function of exhalation , we need no 
other apparatus than half a dozen clean tumblers to try three 
little comparative experiments. Let us call the three ex¬ 
periments A, B, C, respectively. Each requires two glasses ; 
fill one of each pair two thirds with water, the other to be 
left empty. Place a card over each of the three tumblers which 
contain the water so as to cover it, and then invert the empty 
tumbler over the card. Now make a thick paper cap to one 
of the glasses, say B ; lastly, drill a small hole in each card to 
admit the stalk of a leaf, which should dip into the water in 
the lower tumbler, while the blade of the leaf is enclosed in 
the inverted tumbler, the card cutting off the communication 
between the two tumblers in each experiment. Place A and 
B in the direct rays of the sun, B with its cap on. Place C 
in clear daylight, but in the shade. 
In five minutes the empty tumbler A will be clouded with 
dew, but in B and C as yet no dew has been deposited ; the 
dew will now be seen to increase rapidly in A, in C very 
slowly, in B none will be found. Now A, in the direct rays of 
the sun, imbibes and exhales water rapidly ; C, exposed only 
to ordinary daylight, very slowly; and in B, in the dark , it 
ceases altogether. 
It is obvious in these experiments that the light of the sun 
is essential to enable the leaf to perform the function termed 
exhalation. 
It has been ascertained that a (c sun-flower/ ? six feet high, 
will exhale forty ounces of water in the course of a single day. 
Plants absorb by their roots not only the water, but what¬ 
ever matters the water may hold in solution—salts, earth, &c. 
These are retained in the plant. Thus we may understand 
how such substances gradually accumulate in their tissues. 
Perhaps I cannot make you perfectly understand the nature 
