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GUARDIAN OF EXPERIMENTAL SCIENCE.
ROYAL VICTORIA GALLERY, MANCHESTER, On the Chemical Relations now subsisting between Plants and
Animals, with reference to those which have subsisted in former ages.
In a Lecture delivered at the Conversazione, on Wednesday, March 9th, 1842. By Dr. Lyon PLAYFAIR.
It is not an idle task to cast a retrospective glance to ages far beyond human ken, nor to try to discover the physical and chemical causes which then regulated the production and maintenance of animal and vegetable life. Races of animated beings once lived and performed their destined functions on the earth, but they have now passed away for ever. Ages rolled on, and other races were called into being, which have again disappeared, and yielded their places to new forms of organic life. Geologists have examined their remains, and endeavoured to form conceptions of the physical characters of the globe during their residence on it. They have drawn legitimate conclusions, from the order of their occurrence, regarding several grand eras in the history of the world. These inferences have been deduced from external characters possessed by animals and plants occurring in particular formations; and, by a happy application of analogical reasoning, they have shewn what must have been the aspect of the face of nature when these plants and animals existed. Why, therefore, should the chemist be afraid to follow in their footsteps, and by evidences drawn from the chemical composition of matter, lend his aid in explaining the mighty revolutions which have taken place upon the surface of the earth ? True, his science is not speculative, nor does he love to waste his time in the vagaries of theoretical speculation ; but, we believe, that the evidences existing on the earth are sufficient to form a basis for inductive reasoning, without the necessity of substituting ideas for facts.
I shall, therefore, endeavour to lay before you this evening, an Ann. of Elec. Vol. VIII. No. 46. April, 1842.
account of some of the chemical causes which have led to the developement of the various races of animals and vegetables, which have appeared and disappeared in their course. And be it remarked at the threshold of our enquiry, that by the term chemical causes, we do not at all mean to undervalue the physical causes which have lent their aid or been paramount, in their developement or extinction, but merely employ the term to form a boundary in the examination of a boundless subject.
But, before I can make myself understood, it is absolutely necessary that you should be acquainted with the laws which regulate the nutrition of the vegetables now existing on the earth. Mr. Ransome, in a series of lectures, has so ably performed this task, that little remains for me, except to refresh your memory with a few of the grand laws connected with vegetable nutrition. You are all aware that organic matter, in general, is composed of four substances, carbon or charcoal, oxygen, hydrogen, and nitrogen. You know, that when charcoal unites with oxygen, it forms a gas familiar to you as the air which escapes in small bubbles from champagne or beer, and which has received the name of carbonic acid gas. You know also that when oxygen unites with hydrogen, the familiar substance, water, is produced; and that when the latter element (hydrogen) enters into union with nitrogen, ammonia or hartshorn is the product.
Now, these three compounds—carbonic acid, water, and ammonia -form the food of plants. They all exist in the air, from whence they are extracted by the vegetation which covers the earth. The grand difference between animal and vegetable life is, that this carbonic acid, which is fatal to animals, is the primary food of plants ; and that whilst plants are continually inspiring this noxious gas, animals are as constantly expiring it. There are certain constituents in the food of man and animals, which are not at all calculated to assist in the nourishment of the body, but which are, nevertheless, indispensable in supporting the process of respiration. Such are starch, sugar, and gum. I had the honour, on a former occasion, of explaining this to you in detail. These substances, in supporting respiration, are converted into carbonic acid and water. Hence it is that animals continually expire this gas. But there are other substances also exclusively adapted for the nutrition of the system, and which, when present in superabundance, are separated as excrementitious matter. In such substances nitrogen abounds. Hence during their decay, ammonia is generated. When animals die, their bodies enter into a state of putrefaction, or, to speak more correctly, the constituents of which their bodies are composed, change their form, and are converted into carbonic acid and ammonia.
Conceive the thousand millions of men who inhabit the globe, and the myriads of animals which teem on its surface—all sending into the air, every day, vast quantities of that noxious gas. Conceive the vast amount of fuel constantly generating the same com
pound, and it is obvious that the earth would soon become uninhabitable were there no means of removing it from the atmosphere. Nor is this all. As carbonic acid is fatal to animal life, in as great a degree is the oxygen of the air necessary for its support. But this oxygen is always withdrawn from the air as carbonic acid is formed; for carbonic acid consists of carbon and oxygen. Thus, in burning, ten cwt. of coal consumes 32,000 cubic feet of oxygen gas; so that this town of Manchester (calculating its inhabitants at 300,000, the round number of the former census) consumes, for domestic purposes alone, exclusive of the manufactories, no less than 23,614,285,714* cubic feet of oxygen, and sends into the air a like quantity of pestiferous carbonic acid in its stead. Again, each man consumes and corrupts, every day, twenty cubic feet of air ; hence the population of Manchester, by the air consumed in breathing, corrupt 2,190,000,000+ cubic feet of air every year.
But vast as these quantities may appear, let us consider what a fraction of the globe we form; and not to extend our ideas beyond what they will readily embrace ; let us calculate what 100 million men (an insignificant portion of the population of the globe) will annually consume and corrupt of air.
One hundred million men, then, will render unfit for the support of animal life, every year, no less than 9,505,200,000,000 of cubic feet of air.
From such data as these, it is evident, that the air would soon be rendered unfit for the support of animal life were there no means by which it is retained in a state of purity.
This it is the duty of plants of perform. An All-wise Creator has connected the life of plants and animals most closely together, the one depending upon the other. It is, indeed, a wonderful link which associates the vegetable and animal kingdoms. Plants form the primary nutriment of all animals; for although there are certain kinds entirely carnivorous, still the herbivorous animals upon
which they subsist receive their nourishment from vegetable matter. But, following the chain in its continuation, we discover that animals, on the other hand, furnish the food of plants. During their life, they constantly expire carbonic acid, and discharge from their system matter, which by its decomposition, emits ammonia into the atmosphere ; whilst at their death, their bodies decay, and furnish the same substances to the surrounding air. Thus they afford food for
The calculation is as follows:- It has been found that a small town of 7000 inhabitants consumes in fuel, for domestic purposes, 551 million cubic feet of oxygen. 551,000,000 X 300,000
7000 † The calculation is as follows:-Supposing that a man consumes 20 cubic feet of oxygen, each day, in the process of respiration, then 365 X 20=7300. Again, 7300 X 300,000=2,190,000,000.
those very plants, upon which they themselves subsist; thus also the destruction of an existing generation forms the means for the production of a new one, and death becomes the source of life.
Such is a brief outline of the intimate connection subsisting between the animal and vegetable kingdoms. So closely, indeed, are they bound together, that, in the present system of things, we could not conceive the existence of one without that of the other.
But very different seems to have been the arrangements which prevailed in former ages.
Animals were not then created in number sufficient to supply the food of plants. And although we cannot affirm, or even suppose, that the animal kingdom was independent of the vegetable, we have evidence enough to show, that the latter was in no wise dependant for support upon the former. If it be allowed that the paucity of animal remains, even in the carboniferous strata, where plants so abound, proves that the balance betwixt animal and vegetable life now subsisting, was then unknown, we must suppose that the food of plants was then supplied from other
It matters not what vague speculations have been held regarding the cosmogony of the world. With these the chemist has nothing to do.
His science treats only of matter and its properties. Nor can he, without ample demonstration, listen to specious notions propounded of the transformation of the elements of which it is composed, or admit these as a basis for induction. His science teaches him that there are fifty-five bodies on the earth, by the different combinations of which all the varieties of matter are produced ; and further, in the present state of his knowledge, he must allow that all these elements were on the globe when its present position was assigned to it.
However different in intensity and in form may have been the causes which produced the mighty yet gradual revolutions of former times, we discover in them a close analogy to those of the present day. Hence we are not warranted in presuming that the food of plants and animals was different then from what it is now. So far only can we affirm, that the food of plants was received from other sources than at present.
The food of plants consists of water, carbonic acid, and ammonia. What proofs can we find of the existence of these in former periods of the earth's history? During the deposition of the primary strata, where as yet no traces of plants and animals have been found, it is evident that carbonic acid existed. The primary limestones furnish sufficient evidence of its existence. It is true that the partial and irregular distribution of these limestones prove that their deposition was due to limited and local causes ; but this does not militate against the idea that the carbonic acid must have been generally diffused throughout the atmosphere. All the limestones of aqueous origin have evidently been deposited from a solution in water containing an excess of carbonic acid ; for without this ex