THE SOURCES OF THE NITROGEN OF VEGETATION. , ETC. 481 
slowly; so that somewhat rapid variations in the”proportion of carbonic acid in the 
atmosphere surrounding the plant, will be accompanied by much less variation in the 
proportion of carbonic acid within the plant. The latter will, therefore, be a slightly 
varying mean between amounts corresponding to the foregoing extremes. 
From the above considerations, it appeared probable that there would be no danger 
in so supplying carbonic acid to the atmosphere of the plants as that its proportion 
should reach its maximum in a short time, and then, by the passage of air, gradually fall 
again to the minimum. A few trials, adding different quantities of chlorhydric acid to 
the vessel T, Plate XIII. (or T’, fig. 1, Plate XIV.), containing marble, enabled us to 
ascertain the proper quantity to add, to provide about 4 per cent. of carbonic acid in the 
shade F when air was not passing. Then passing air, it was found that the proportion 
of the gas was never reduced below that which we have above assumed as the proper 
minimum. In practice a little more chlorhydric acid than the amount so determined 
was used; and then the passage of the air was commenced simultaneously with the 
addition of the acid. Repeated analysis of the air in the enclosing apparatus showed 
that, operating in this way, our assumed limits for the maximum and minimum propor- 
tions, respectively, of carbonic acid were not passed. 
The volume of the air passed through the apparatus daily, was that of the vessel A, 
Plate XIII. and fig. 1, Plate XIV., and was equal to about 2°5 times that of the 
enclosing shade F. ~~ 
K.— Advantages of the Apparatus above described. 
The advantages, for the purpose in question, of the plan of apparatus which has been 
described, over those of several of the forms that have been suggested or used by others, 
may be very briefly stated. 
1. When once ready to receive the plant, the use of the apparatus is extremely simple 
and easy. It is only necessary to place the pot containing the soil, seed, &c., with its 
pan, in the stone-ware lute-vessel, to pour mercury into the groove, to arrange the several 
tubes, and to put on the shade. The plant is then entirely excluded from all external 
sources of combined Nitrogen; and, in case of its being nece§sary to open the vessel 
for any purpose, this can be done with great facility. 
2. By means of the arrangement of the bottle O (fig. 1, Plate XIV.), the water which 
condenses within the shade is removed from the atmosphere of the plant as soon as it 
collects. ‘The small pan in which the pot stands (fig. 3, Plate XII.), with its inward- 
turned sides, allows of a store of water being kept beneath the plant which is at the 
same time protected from free evaporation. The vessel O holds as much water as can be 
evaporated from the plant and soil during several days. The supply of water to the 
plant is exceedingly easy and simple, it being only necessary to remove that which has 
collected in the bottle O by means of the tube ¢’, and to pour it in at w (figs. 1 and 2, 
Plate XIV.). [In the arrangement for the experiments of 1857 the condensed water 
collected on the surface of the slate, until removed by means of the tube n 0.] 
3 U2 
