THE SOUECES OE THE XITEOGEX OE VEGETATION, ETC. 483 



to pass through the solution of carbonate of soda in the bottle E, and 



1 x 1 "So =1-85 inch of water 



to pass through the sulphuric acid in the bulb-apparatus M, equal a total of 14-10 inches 

 water, or a minimum pressure of about 0-5 lb. per square inch. Direct experiment 

 with a manometer showed, however, that the entire pressure, minus that due to the sul- 

 phuric acid in the bulb-apparatus M, might, owing to friction, &c, amount to 0-8 lb. per 

 square inch. This would give a lateral pressure upon the sides of the glass shade of about 

 900 lbs., if the current of air were produced by aspiration instead of forcing — a condition 

 which would be incompatible with the safety of the vessel. In the mode of experiment- 

 ing adopted, however, the only pressure exerted upon the glass shade was the amount 

 requisite to force the air through the bulb-apparatus M. 



It remains to consider the influence upon the air of its contact (in the vessel A) with 

 the water employed to force it through the apparatus. This can be of three kinds : — 



1. The proportions of nitrogen and oxygen may be slightly affected by absorption, 

 under the influence of the slightly increased pressure to which the air is subjected. 



2. The air may lose its carbonic acid. 



3. It may become more or less saturated with aqueous vapour. 



The increase of pressure to which the air is subjected in the vessel A is so slight, and 

 the time in which it is there in contact with the water is so short, that the total amount 

 of oxygen and nitrogen absorbed by the water must be very small; and, since any 

 change in the constitution of the total amount of air will be dependent on the ratio of 

 the absorption coefficients of oxygen and nitrogen on the one hand, and on the ratio of 

 the quantities of these gases in the air on the other, it will be very much less than in 

 the actual amount of air absorbed ; it will in fact be too small to be of any importance. 



The whole of the carbonic acid of the air may be absorbed by the water ; but as 

 arrangements are made for the artifical supply of it, this is of no consequence. 



The amount of water taken up by the air in the vessel A would at first sight appear 

 to be of more importance. But the time during which the air is in contact with the 

 water in the vessel A is very short, and probably too short for its saturation ; it must 

 lose most or all of its acquired water in passing through the sulphuric acid in the bottles 

 B and C, and over the pumice saturated with sulphuric acid in the tube DD, whilst the 

 redried air passes too rapidly through the carbonate of soda solution in the bottle E for 

 re-saturation ; and lastly, as the air in its previous course through the apparatus will be 

 cooler than within the shade, it will not be so near its point of saturation in the latter as 

 it may be before it reaches it. 



L. — Adaptation for healthy growth of the conditions of experiment adopted. 



"We have thus far discussed the possible sources of error in an experiment on the 

 question of the assmilation of Nitrogen by plants, so far as regards the soil, the inorganic 

 nutriment, and the air, to be provided for the plant, and we have pointed out the means 



