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hydrogen, and in nitrogen, as well as in a mixture of nitrogen, carbonic acid, and vapour of water-representative of the "primitive atmosphere" at the time life first appeared.

I was thus gradually led to conclude that the primitive atmosphere of the Earth was nitrogen, into which volcanic action poured more or less carbonic acid and vapour, and that after vegetable life appeared, free oxygen made its appearance in the air, and has increased in quantity from those primitive times to the present day.

I had noticed many years ago that certain plants (willow, lilac, etc.) did not thrive in pure carbonic acid, and I concluded that the diminished quantity of this gas now existing in our atmosphere was the condition that suited them best.

In experiments carried out more recently, I have placed various other plants, such as Poa, Agrostis, Myosotis, Antirrhinum, and Convolvulus in an atmosphere composed of pure carbonic acid, and in one composed of air with about a hundred times more of this gas than exists in our atmosphere at the present day. All other conditions of vegetation were normal: there was ample water, mineral elements, and an appropriate temperature ranging from 59° to 70° Fahr., during the whole course of the observations.

It was not long before I found that my plants could exist for many days, or even weeks, in an atmosphere of pure carbonic acid, but they did not thrive; cell formation became slower and slower, in spite of a constant good light.

In an atmosphere that contained so much carbonic acid that an animal exposed to it would perish in a few minutes, all my plants lived for many weeks, and appeared healthy.

In an atmosphere containing one hundred times as much carbonic acid as in the natural state of the air in our day, my plants flourished remarkably well for the whole time that the experiment lasted-a month, or six weeks.

It was thus rendered highly probable that in former geological periods there existed plants which could flourish in an atmosphere excessively rich in carbonic acid, and that the quantity of that gas in the air has really decreased from those times to the present day; the loss being represented by the vast deposits of peat, lignite, coal, and anthracite found in the strata of the Earth.

I also made some experiments in an atmosphere of pure hydrogen, out of curiosity to see what would happen when plants were placed in such a mediuma gas which many have looked upon as the vapour of a metal-all other conditions remaining normal, namely, water charged with carbonic acid, and containing the necessary mineral ingredients, and a steady source of daylight.

The plants I placed in this atmosphere of pure hydrogen were Convolvulus arvensis (a plant which is very convenient for observation of this kind), and a small specimen of Antirrhinum majus. For the first few days nothing peculiar was noticed. From

the 27th May to the end of the month a slight bleaching of the leaves only was apparent, but by 3rd June a singular phenomenon occurred: the volume of gas began to diminish, and in the course of another month the atmosphere of hydrogen in which the Convolvulus vegetated was absorbed to the extent of about 80 per cent.

The residue, 20 per cent., was found not to be hydrogen; so it may safely be said that the whole of the hydrogen had disappeared, the plant remaining perfectly healthy. I can only explain this phenomenon by assuming that the nascent oxygen emitted by the leaves burns up the hydrogen, and converts it into water. The same thing occurred with the Antirrhinum, but the action was slower. In both cases the leaves were slightly bleached, and as the water charged with carbonic acid rose in the apparatus and covered them, they became green again.

In all these experiments, the plants were exposed to the constant light of a northern sky, such as artists use for painting.

The rapidity of cell formation in an atmosphere of hydrogen gas, observed in the case of Convolvulus arvensis, was highly remarkable.

CHAPTER IV.

The Discoveries made by Priestley-Mutual Dependency of Animals and Plants-The Author's Experiments with Unicellular Alge―The Author's Experiments with Convolvulus arvensis and other Plants.

IN 1791, when Dr. Priestley's house was burnt by the Birmingham mob, and he saved his life by escaping in my grandfather's carriage, a considerable amount of valuable silver plate, etc., was carried off, but none of the still more valuable MSS., the results of years of labour. These were destroyed, together with the entire library and laboratory. It is therefore impossible to say how far he may have pushed his investigations with regard to the mutual dependency of plants and animals after discovering, some twenty years previously, that "a sprig of mint vegetating for a few days in an air vitiated by a burning candle restored the purity of that air sufficiently to allow the candle to burn in it again."

In the seventh edition of the Introduction to Botany, by Sir J. E. Smith, edited by William Hooker (1803, page 104), the author, alluding to the absorption of carbonic acid by plants, and its replacement, volume

for volume, by oxygen gas, says: "This beautiful discovery, for the main principles of which we are indebted to the celebrated Dr. Priestley, shows a mutual dependence of the animal and vegetable kingdoms on each other, which had never been suspected before his time."

It is not probable, however, that Priestley could have made much further advance in this direction, owing to the backward state of chemistry in his day, and to his zeal for political and religious discussions. It was reserved for the persevering researches of the Swiss chemist, Théodore de Saussure, and to the splendid genius of Justus von Liebig, to demonstrate this mutual dependency in all its striking realities.

Yet it is greater than even Liebig imagined!

1

After having found that minute microscopic plants, such as the Protococcus pluvialis and P. palustris (unicellular algæ), could be easily transformed, so to speak, into manufacturers of oxygen gas, and that they produced it more rapidly than do the higher plants of our epoch, I placed several kinds of the latter in pure carbonic acid, and found that it did not kill them at once, as it would kill an animal, but that they lived in it for some time, though they did not prosper. an atmosphere of hydrogen they lived and prospered; but the hydrogen gas was slowly absorbed, as we have already seen, until it had all disappeared. In the next place I exposed my plants to an atmosphere of pure

In

1 1 Phipson, "Production of Oxygen by Protococcus pluvialis" (with figure of the apparatus), Chemical News, 1883; "The free Oxygen of the Atmosphere," ibid., November 1894.

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