Experiments with the electric light have been carried out by Herve-Mangon in France and Dr Siemens in England. The plants grown under the influence of the electric light were observed to be of a lighter green colour than those grown under normal conditions, thus indicating a feebler growth; in fact, Siemens was of the opinion that the electric light was about half as effective as daylight.[23] These experiments are interesting from an industrial point of view; for it is conceivable that at some distant time electricity might be called to the aid of the agriculturist. _Source of Plants' Oxygen._ With regard to the source of the oxygen, which, next to carbon, is the element most largely present in the plant's substance--amounting to, roughly speaking, about 40 per cent--all evidence seems to indicate that it is chiefly derived from water, which is also the source of the plant's hydrogen. In addition to water, carbonic acid and nitric acid may also furnish small quantities. It has been pretty conclusively proved that the atmospheric oxygen, while necessary to plant-growth, and promoting the various chemical vital processes, is not a direct source of the plant's oxygen. The important function played by atmospheric oxygen in certain stages of the plant's growth has been long recognised. Malpighi, nearly two hundred years ago, observed that for the process of germination atmospheric air was necessary; and shortly after the discovery of the composition of the air was made, oxygen was identified as the important gas in promoting this process. Oxygen is also especially necessary during the period of ripening. _Source of Plants' Hydrogen._ Hydrogen, which amounts to about 6 per cent, is, as has already been pointed out, chiefly derived from water. It is possible that ammonia also may form a source. _Source of Plants' Nitrogen._ When we come to treat of the source of the nitrogen, which is found in the plant's substance to an extent varying from a fraction of a per cent to about 4 per cent, we enter on a much more debated question. What is the source, or, what are the sources, of plant-nitrogen? is a question to the solution of which more time and more research have been devoted than to the solution of any other question connected with agricultural chemistry. The most obvious source is the free nitrogen, which forms four-fifths of the atmospheric air. Reference has already been made to this question.[24] Priestley was the first of the long list of experimenters on this interesting question.