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the air contains ozone, ammonia, carburetted and sulphuretted hydrogen, carbon monoxide, sulphurous and sulphuric acids, nitric acid, etc. The ozone and nitric acid are the products of the action of electricity on the contents of the air, especially during thunder storms. The amount of ozone present is often chosen as a test of the healthiness of a region, and that of the carbonic acid of its unhealthiness. Many solid substances, in fine division, float constantly in the air, and can be seen wher a beam of direct sunlight passes through a dark or shaded room. In the struggle for existence among both plants and animals, throughout all time, the atmosphere, through these and other qualities it possesses, has had an influence second, to nothing else, in moulding their forms and directing their destinies.

The primitive atmosphere was a very markedly different one from that described. When the fiery mass of the earth began to crust itself over, the weight, volume, composition and character of the air were totally unlike that of to-day. Every particle of oxygen must have been held in chemical union, since the conditions for such union were present and more than enough oxidizable material within reach. The only part of the atmosphere that would make a world possible for us to inhabit was therefore totally absent. All the water of the oceans was in the air. All the carbonic acid in the mountain ranges and deep strata of carbonates was likewise there. That which we now know as coal must then have existed as carbonic acid. Such an atmosphere, even had it been cool enough to match our temperature, would have been absolutely unfit to breathe, both because it did not contain the element that makes life possible and because it did contain in its stead more than an equal amount of deleterious gases.

The genesis of organic matter must have preceded the production of organisms. Centuries before a crust had formed upon the molten material of the earth, the dance of the carbon atoms had begun and the syntheses of the simpler forms progressed. Amid the terrific tempests and almost ceaseless series of deluges then going on the magic linkings of carbon, oxygen, hydrogen and nitrogen were steadily advancing in

complexity and reaching toward bioplasm. While the hot rock seethed and boiled like a cauldron over a furnace, in quieter and cooler spots additional new and more complex bodies were appearing. At last in the cradle of the deep appeared the infant protoplasmic germ in whose tiny form was locked the promise and potency of all that we can boast of. The earth was

then an enormous piece of cooling slag and the atmosphere an equally forbidding mass of irrespirable poison. Every wind that then blew was a current of choke-damp and the rains as they fell becoming saturated with this gas, descended as deluges of soda water, identical with that drawn by druggists and confectioners from their fountains though perhaps scarcely as pure. No lamp, candle or fire could have burnt anywhere on our - planet. Nothing that breathes could have survived such conditions for a moment. On the barren surface of the fire scarred earth there was no soil to harbor vegetation and no vegetation to be harbored. life, in any form, however rudimentary, was able to develop or exist under conditions such as then obtained, it is difficult to understand. In an atmosphere that must have been almost or totally devoid of free oxygen how did protoplasm oxidise? Without oxidation of the substance constituting living things, life, as we know it, would be impossible. Late studies in bacteriology show us that many micro-organisms have the power of decomposing carbon dioxide and setting oxygen free independent of sunlight. The writer, in a paper read at the last New York meeting of the American Association for the Advancement of Science, showed that some of the higher fungi possessed this power and in such medicinal substances as dilute phosphoric acid were able to increase in weight without reducing the quantity of acid present in any perceptible degree. Prof. Farley of Harvard, our leading American mycologist, at that time doubted the accu racy of my results; but a number of German experimentors have since confirmed them on other species. It is now quite certain that many if not all fungi possess this power. At quite an early date in the study of micro-organisms Pasteur divided them into aerobic or air requiring microbes and anaerobic or such as could grow in the absence of free oxvgen.

All the facts at our command concerning the early conditions of our planet force us to conclude that its first life was anærobic. The fogs, clouds and heavy atmosphere of those times forbade the struggling sunbeams admittance, so that "darkness was upon the face of the deep." Without free oxygen and in obscure gloom, living matter was forced to exist and begin the work of preparing this planet for us. Little by little oxygen was released from its bonds with other substances and as a free element kept increasing in amount and thereby making possible higher forms. of life. At first the changes could not be other than slow. The life activities must have trod a pace so slow that our metaphor of "a snail's pace" would really over-estimate the same. The lower down we go in the biological scale the more sluggish is the metabolism of tissue found to be. Such sluggishness is incompatible with rapid adjustments to changing conditions. It was, however, the best that could then be found. The rapid, spontaneous and purposive release of energy from a body requires molecules of a certain definite structure, the most noticeable feature of which is the amount of nitrogen they contain. characteristic of combined nitrogen is In all of its compounds it acts like a that ever seeks to be released. In its free state, on the contrary, it is so stable that it is almost impossible to discover anything but negative properties with which to describe it. It is the very type of impassivity. Chemically, therefore, oxygen and nitrogen are almost opposites in characteristics. Nitrogen can only with difficulty be made to unite with anything, and once united it appears to be incessantly seeking release. Some of its well known compounds will rapidly become disrupted by a spark or blow. We all know how many accidents have occurred from the explosion of nitroglycerine or dynamite by merely a jar or blow. All of our dangerous explosives contain this element and owe their tremendous power to its desire for freedom from chemical bondage. The chloride, bromide and iodide of nitrogen are the most powerful explosives known, but their instability is so marked they are utterly useless. A mere jar will send off

The peculiar its instability. coiled spring

chloride of nitrogen with fearful and uncontrollable violence. Of course the power released is proportional to the amount going off. If we could separate the millions of molecules composing a few grains of it and have them explode one by one under the excitement of divers forms of stimulating jars, we should duplicate the conditions of life energy. When chloride of nitrogen suddenly explodes with merely a light jar, or even spontaneously without visible cause, it shows in a coarse or crude way what happens in a cell when a very slight stimulus makes it betray the fact that it is alive. In both cases molecules containing nitrogen are disrupted. Increase of nitrogen in living structures always signifies increased sensitiveness to external stimuli. This holds good throughout animated nature. The vegetable world displays less power of adjustment than the animal and shows a corresponding reduction in amount of nitrogenous. material in its structure. The higher up the animal scale we travel the larger the amount of nitrogen that enters into the structure of the organism in proportion to weight or volume. In any given animal the higher the function of any tissue that enters into its structure, the larger the amount of nitrogen. The bones have the least and the nerve structure the most, while the brain has far more than any other part. As a mere rub or slight blow explodes chloride of nitrogen, so an image formed by light on the retina of the eye explodes our cerebral molecules and lets loose the energy we call will power. The break-up initiated among the nitrogenous molecules of the brain by causes so light, imparts energy to the coarser molecules containing more carbon and hydrogen. The breaking up of these requires oxygen, and this is supplied by the iron in the blood that holds this eleinent in loose combination. In highly differentiated animals whose nervous structures contains a maximum of nitrogen, these decompositions are most numerous, and hence they require more oxygen. For this reason cold blooded animals can live where the warm blooded would perish. The combustion of their tissues is exceedingly sluggish. In plants the changes are still more slow. Plants likewise differ from animals in their

ability to produce; usually in the presence of direct sunlight, more than sufficient oxygen, from the carbon dioxide of the air, to supply their own requirements.

The breathing of plants and animals is the same. Not long since it was believed that animals only required oxygen, and that plants required carbon dioxide. This was an error. Plants do not breathe carbon dioxide, but, like ourselves, oxygen. They, however, find by far the largest volume of their food in the carbon dioxide of the air. This they decompose by aid of direct sunlight, and by seizing upon the nascent carbon convert it into plant tissue, while the oxygen escapes into the atmosphere. Since they thus release far more oxygen than they require for themselves, the air is kept constantly pure by their good services. Because they provide an oxygen balance of this kind and have ever done so, to them we are indebted for the character of our present atmosphere and its most marked difference from that of primitive times. They have, during countless millenniums, been free cxygen manufacturers. Beginning their work in an atmosphere that was oxygenless, they have gone on with their good services until it is now fit for the breathing of healthy human beings. The change from the wholly irrespirable air of early times to the perfectly respirable air of to-day has come through slow, insensible degrees. Step by step it has been transmuted into the fitness that has made possible warm blooded, quickacting and quick witted creatures. Had the work of

plants not preceded us we could never have been; and should they stop such work it would mean speedy extinction for us. Our lives are bound up in theirs and theirs in ours by a reciprocity so perfect that it is marvelous. Imagine two sets of machines so adjusted that running down of one winds up the other, and that the the stopping of either stops both, and you will have a partial conception of the intimate relationship of the two. Our breathing and the fires of our homes and workshops supplies them with their daily food, and they supply us not only with food and clothing, but with pure air as well. Had the vegetable world succeeded in converting all the carbonic acid gas of the primitive atmosphere into oxygen and carbon the pro

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