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the change referred to-a cell which combines the plant and animal properties, and secretes carbonic acid in the conditions in which the green unicellular algæ secrete oxygen. In the latter case, the oxygen evolved is separated from carbonic acid, and the carbon retained by the plant; in the former, carbonic acid is secreted by the yeast cell just as in the higher animals; the function is far more complicated, and requires a special nutriment, and the existence is rapid and short.

The French naturalist, A. Dissard, has recently published a paper, La transpiration et la respiration chez les Batraciens (Comptes-rendus, Paris), which gives us some idea how the change in the medium determines a change of function, such as I allude to here. Respiration is more active with aërial batrachians than in the aquatic species; exactly the reverse occurs for transpiration.

In one of my experiments, in which a Convolvulus grew for fourteen weeks (25th July to 30th October 1893), the confined" primitive atmosphere" lost all its carbonic acid, and the atmosphere at the end of the experiment was richer in oxygen than is ordinary atmospheric air. This shows what would happen to the Earth's atmosphere if there were an excessive supply of carbonic acid, and vegetation did not deteriorate: the oxygen of the air would increase year by year.

In the present state of things there is a kind of equilibrium apparent (not real), as during the last fifty or sixty years no great excess of oxygen gas has been detected by chemical analysis of the air, in whatever

locality, or at whatever elevation over the sea it may happen to have been collected. But what are fifty or sixty years compared to the thousands of centuries by which Nature counts her periods !

At Palermo it has been found that there is a slight diminution of oxygen in the air whilst the Sirocco blows. At Dresden, Professor Ficinus found a slight variation according to the direction of the wind; with the west and south winds the quantity of oxygen was always the highest. Something similar has been noted with regard to air analyzed at Copenhagen, at Helsingfors, at Guadaloupe, and of the air over the North Sea. In spite of the utmost care in the analyses, there were considerable differences in the amount of oxygen found in all these cases. The air of Calcutta during an outbreak of Cholera in 1845 yielded only 20-35 of oxygen (instead of 21), and 0.13 of carbonic acid (instead of 0.04 or 0.05); and there are other examples tending to show that the volumetric proportion of 79 of nitrogen to 21 of oxygen is anything but a fixed quantity. However, I have little faith in the accuracy of these analyses; and they must always be reduced to 0° C. and 30 inches barometer, which some of them were not. In former years also, analysis of the air was very frequently performed with phosphorus, which gives extremely erroneous results unless special precautions are taken.

In concluding the first part of this little work, we cannot help observing that it is a striking reflection when we consider that all the superiority of Man upon

this Earth depends upon fire, and that this fire is got by oxygen, originally the product of the minute cells of such humble plants as the microscopic unicellular algæ, vegetating under the mysterious agency of the Sun's rays!

Light, indeed, may be said to be the origin of Life.




The Atmosphere is only a Mixture of Gases, not a CompoundIrrefragable Proof of this by Berzélius-Its Changeable Composition-The Inert Nature of Nitrogen-Ammonia and Nitric Acid-The Author's Experiments on their Mutual Conversion-The Unexplained Phenomenon of "Nitrification "-Its Universality-Ammonia, like Carbonic Acid, a Volcanic Product-Nitrification due to the Oxidation of Atmospheric Ammonia.

THE ancients looked upon the air as an element, and I have shown that in the earliest ages of the globe it was really so, for the atmosphere must then have consisted of nitrogen only, and the free oxygen which now forms part of the air we breathe, and without which animal life is impossible, is entirely the product of plant life extending over countless ages.

At the present time the atmosphere of the Earth, in accordance with the foregoing considerations, is not a compound, but a mixture of nitrogen and oxygen. This was amply proved by the great Swedish chemist, Berzélius, in his Traité de Chimie (vol. i. p. 88, Belgian

edition). Here is the passage in question; it constitutes an important page in the history of science :

"Some naturalists have endeavoured to prove that atmospheric air is an oxide of nitrogen. They base their opinion chiefly on the fact that it is almost exactly composed of four parts of nitrogen for one part of oxygen, and that consequently it contains half as much oxygen as nitric oxide gas. But if such were the case, atmospheric air would offer to us the first known example of a simple mixture having absolutely the same properties as a chemical combination of the same elements. For, in fact, an artificial mixture of four parts of nitrogen and one of oxygen does not differ from atmospheric air as to its physical and chemical properties; and what proves clearly that this mixture is not a chemical compound is that no change in the volume, nor of the temperature, takes place at the moment the mixture is made. As, moreover, nitric oxide gas is converted into nitrous acid when it comes in contact with the air, it would result from this opinion that an oxide of a higher degree, and containing more oxygen, would possess the property of reducing, without the co-operation of any third body, an inferior degree of oxidation of the same radicle, a case of which Chemistry offers no example. Atmospheric air is, therefore, not a gaseous oxide of nitrogen, but a simple mixture of nitrogen and oxygen gases."

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This settled the question for ever. And it was this knowledge that our atmosphere is but a mixture, and


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