THE SOITRCES OF THE XITKOGEX 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 

 shoidd 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. Eepeated analysis of the ah 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 necessary to open the vessel 

 for any purpose, this can be clone 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 t', and to pour it in at u (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 o.] 



3 u2 



