RAIN WATER DISSOLVES IT. gg 



have as yet no experimental results from whicli we can form any esti- 

 mate. Whether it lakes place at all or not, is wholly a matter of opinion. 



2°. The leaves ofplants, as will be more fully explained hereafter, absorb 

 certain gaseous substances from the atmosphere, and we might, therefore, 

 expect that some of the nitrogen of the air would, by this channel, be 

 admitted into their circulation. This view, however, is not confirmed 

 by any of the experiments hitherto made with the view of investigating 

 the action and functions of the leaves.* We are not at liberty, there- 

 fore, to assume that any of the nitrogen which plants contain has in this 

 way been derived directly from the air. It may be the case ; but it is 

 not yet proved. 



3°. There is little doubt, however, that nitrogen enters the roots of 

 plants in a state of solution. But the quantity they thus absorb is un- 

 certain — it is supposed to be small, and must be variable. 



When water is exposed to the air in an open vessel it gradually ab- 

 sorbs oxygen and nitrogen, though, as has been stated in a previous lec- 

 ture, in proportions different from those in which they exist in the atmos- 

 phere. Tlie whole (juantity of the mixed gases thus taken up amounts 

 to about 4 per cent, of the bulk of the water (Humboldt and Gay-Lus- 

 sac), and in rain water about | of the whole consist of nitrogen. One 

 hundred cubic inches of rain water, therefore, will carry into the soil 

 about 2| inches of nitrogen gas. But in passing through the soil, the 

 water meets with other soluble substances before it reaches the roots, 

 especially the deep-seated roots of plants. It takes up carbonic acid, 

 and it dissolves solid substances, and in doing so it is a property of water 

 to give off a portion of the other gases which it had previously absorbed 

 from the air. 



But let us suppose that rain water actually takes to the roots, and car- 

 ries with it into the circulation of the plant, 2 per cent, of its bulk of 

 nitrogen, and let us calculate how much of the nitrogen it contains a 

 crop of hay could in this way derive from the air. 



* See subsequent lecture " On the structure and functions qftlie several parts of plants.^' 

 Tlie experiment.s above referred to were made upon plants growing in close vessels, the 

 air contained in which was measured and examineo (analysed) both before the plants were 

 introduced and after they had been some time in the vessel. In these expedments the 

 bulk of (he nitrogen present has sometimes been observed to increase, but never to dimin- 

 ish^ in quantity. The conclusion seems satisfactory, that no nitrogen is abstracted directly 

 from the atmosphere by the loaves of plants. Yet Boussingault^ very justly remarks, that 

 a diminution in the bulk of the nitrogen too small to be detected in the ordinary mode of 

 making these experiments, would be sufficient to account for a considerable portion of tliat 

 comparatively s-raall quantity of nitj"ogen which is present in all living plants. While, there- 

 fore, we accord their due weight to these researches of the vegetable physiologists, we are 

 not to consider them as by any means decisive of tjie question. With this rational and cau- 

 tious conclusion, Liebig is not satisfied ; he says, " We have not the slightest reason for be- 

 lieving that the nitrogen of the atmosphere takes part in the processes of assimilation of planta 

 and animals ; on the contrary, we know that many plants emit the nitrogen which is ab- 

 sorbed by their roots either in the gaseous form or in solution in water." (p. 70.) But if 

 they occasionally expire nitrogen by their leaves why must this nitrogen be exactly that 

 portion which has previously been absorbed by the roots in the uncombined state, aiid the 

 quantity of which is so unceitain and so indefinite? 



r* Boussingault details a series of experiments in the course of which he made peas, tre- 

 foil, wheat, and oats, grow in the same pure siliceous sand containing no organic matter, ana 

 watered them with the same distilled water. The absolute quantity of nitrogen increased 

 sensibly in the peas and trefoil during their growth ; in the wheat and oats no change could 

 be detected by analysis. From these results he is inclined to infer that the green leaves oi 

 the former have the power of sensibly absorbing nitrogen from the atmosphere, while those 

 of the latter have not this power — at least under the circumstjuices in which the experi- 

 ments were made. This conclusion, however, is not certain^ as will presently be shewn.— 

 See Aim. de Chim. el de Pkys. Ixvii. p. 1, and Ixix. p. 353.] 

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