476 MR. J. B. LAWES, DR. GILBERT, AND DR. PUGH ON 
As in the case of the soil, so in that of the atmosphere, the only essential conditions 
to be attained are such as are required for healthy growth, and as will at the same time 
enable us to estimate the amount and the sources of the combined Nitrogen coming 
within the reach of the plant by its means. 
In consequence of the mobility of the atmosphere above referred to, it was necessary 
to exclude the experimental plants from its free access. The quantity of ammonia in 
the air is, however, so very small, that, provided the atmosphere of the enclosing apparatus 
were allowed to remain unchanged throughout the period of an experiment, the amount 
of combined Nitrogen so coming within the reach of the plant might be altogether 
neglected. Nor, so far as regards the nitrogen and oxygen of the air, is there any 
necessity for change; but, owing to the peculiar circumstances of temperature and of 
moisture to which the air of the apparatus is subjected, conditions more closely allied 
to those of ordinary vegetation are attained by a frequent change of atmosphere. The 
large quantity of air which thus becomes involved in an experiment precluded the idea 
of neglecting the consideration of the combined Nitrogen which it contains. It devolved 
upon us, therefore, either to determine the total amount of combined Nitrogen in the 
air before and after it came in contact with the plant, or to free the air from combined 
Nitrogen before admission into the enclosing apparatus. The latter alternative was 
adopted as the most simple ; and the manner in which the object was effected will appear 
from the following description of the apparatus employed. 
H.—The Apparatus used to enclose the Plants, and to supply them with Air, Water, 
: Carbonic Acid, &e. 
Plate XIII. represents the entire apparatus as used for each separate experiment in 
1857; and fig. 1, Plate XIV., that used, also for each separate experiment, in 1858, in 
which, as will be seen, several important modifications of the arrangement adopted in 
1857 were made. 
The same letters of reference apply to the two so far as the parts are alike; and 
where there has been any modification in the arrangement in 1858, as compared with 
that in 1857, the same letters represent the parts of the apparatus used for the same 
purpose in each, with the exception, that those which apply to the modification of the 
apparatus in 1858, are distinguished by a dash, thus '. 
A, Plate XIII. (and fig. 1, Plate XIV.), represents a large stone-ware Woulfe’s bottle, 
18 inches in diameter and 24 inches high. 
B, C, and E are glass Woulfe’s bottles of 30 ounces capacity. 
F is a large glass shade, the dimensions of which were, in most of the experiments, 
diameter 9 inches, and height 40 inches; in other cases the dimensions were, diameter 
16 inches, and height 28 inches. 
a represents the cross section of a leaden pipe 14 inch in diameter, which is in con- 
nexion with a reservoir of water, not shown. This pipe passes over all the vessels A (of 
