SOURCE OF THE HYDROGEN OF PLANTS. 65 



tion with oxygen. In the form of vapour, this compound pervades the 

 atmosphere, and plays among the leaves of plants, while in the liquid 

 state it is diffused through the soil, and is unceasingly drunk in by the 

 roots of all living vegetables. In the interior of plants — at least during 

 their growth — this water is continually undergoing decomposition, and 

 it is unquestionably the chief source of the hydrogen which enters into 

 the constitution of their several parts. In explaining the properties of 

 water I have already dwelt upon the apparent facility with which its 

 elements are capable either of separating from, or of re-uniting to, each 

 other, in the vascular system of animals or of plants. The reason and 

 precise results of these transformations we shall hereafter consider. 



2°. In light carburetted hydrogen (CH2), given off as already stated 

 during the decay of vegetable matter, and said to be always present in 

 highly manured soils, this element, hydrogen, exists to the amount of 

 nearly one-fourth of its weight. On the extent, therefore, to which this 

 gaseous compound gains admission into the roots of plants, will de- 

 pend the supply of hydrogen which they are capable of drawing from 

 this source. Had we satisfactory evidence of the actual absorption of 

 this (marsh) gas by the roots or leaves of plants, in any quantity, we 

 should have no difficulty in admitting that plants might, from this source, 

 easily obtain a considerable supply both of carbon and of hydrogen. It 

 would be also easy to explain how (that is, by what chemical changes,) 

 it is capable of being so appropriated. But the extent to which it really 

 acts as food to living vegetables is entirely unknown. 



3°. Ammonia is another compound, containing much hydrogen, [its 

 formula being NH^, or one equivalent of nitrogen and three of hydro- 

 gen,] which, as I have already stated, exercises a manifest influence on 

 the growth of plants. If this substance enter into their circulation in 

 any sensible quantity, — if, as some maintain, it be not only universally 

 diffused throughout nature, but is constantly affecting, and influencing at 

 all times, the universal functions of vegetation — there can be no doubt 

 that the hydrogen it contains must, to an equal extent, be concerned in 

 the production of the various organic substances which are formed or 

 elaborated by the agency of vegetable life. How far this probable in- 

 terference of the hydrogen of ammonia with the functions of the vegeta- 

 ble organs, will tend to explain or illustrate the influence actually exert- 

 ed by this compound, we shall, by and by, more accurately inquire. In 

 the mean time, the quantity of ammonia, which actually enters into the 

 circulation of plants in a state of.nature, is too little known, and making 

 the largest allowance, probably too minute, to permit us to consider it as 

 an important source of hydrogen to the general vegetation of the globe. 



4°. The soluble organic substances, which enter into the circulation 

 of plants through the roots, as shewn in the preceding section, do not 

 consist of carbon and water only, but of combinations of carbon with 

 hydrogen and oxygen in various proportions. From these substances, 

 therefore, plants derive an uncertain and indefinite supply of hydrogen 

 in a state already half-organized, and probably still more easily assimi- 

 lated or converted into portions of their own substance, than when this 

 element is combined with oxygen in the form of water. 



We may, therefore, conclude generally in regard to the source of the 

 hydrogen of plants — that though there are undoubtedly several other 



