CAUTION IN CHEMICAL REASONING. 123

thing is plain. This is the cause of animal "heat!"

It has always struck us as a curious thing that chemists should have attached such a dominant influence in the production of heat in animals, to the union of carbon and oxygen; because nobody is necessarily so familiar as they are, with the fact that the evolution of heat is not at all peculiar to the union of these bodies; but is a circumstance common to all changes of every kind, in all forms of matter; there always being either the absorption or the evolution of heat.

There is no doubt that the analogy is very striking between the changes which appear to be wrought in respiration, and those which take place in ordinary combustion. A very little consideration shows that the idea that respiration is the cause of animal heat, or that it is due to any other change of oxygen, is not only an assumption; but in the highest degree doubtful. In the first place, the carbonic acid thrown out when we expire is certainly not made by the immediate union of oxygen with charcoal ex

124

RESPIRATION, WHAT.

pired; secondly, nothing is so clear that in respiration there is an immense quantity of heat thrown out of the body. But as it is very desirable that the subject of this paper of Abernethy's on the Skin and Lungs should be understood, we will give the reader a simple view of the nature of these important organs; and as one (functionally considered) is as much a breathing organ as the other, we will say a few words first of the lungs.

In all animals, the blood, or other fluid in which the elements of nutrition are sent to all parts, is exposed to the action of the air, and this is what we call breathing or respiration; and the exposing of the blood to air is so arranged that both fluids are in more or less rapid motion. The staple constituents of the air, so to speak, are about one-fifth oxygen and four-fifths nitrogen gases, with about two parts perhaps in a thousand of carbonic acid; and although, as we too well know, the air is occasionally polluted by many additions, yet whether we take air from the top of Mont Blanc, or a cellar in London, the staple principles of oxygen

OBJECT OF HIS INQUIRY.

125

and nitrogen have their proportions unchanged. The air breathed by animals, who live in the water is somewhat differently constituted; the proportion of oxygen is considerably greater, probably about as much as one-third or thirtytwo parts in one hundred; so that fish breathe a more highly oxygenated air than we do..

Now it is found that when we inhale the air of the atmosphere, (that is to say, one-fifth oxygen and four-fifths nitrogen), that we expire some oxygen, some carbonic acid, and some nitrogen also; and to ascertain the actual changes which took place, was the object of Abernethy's inquiry.

The subject is one of great interest to the public, and in justice to Abernethy we should remark that this essay was written more than half a century ago—1793.

Thousands die every year of affections of the lungs; and many diseases of these organs, if not in their nature incurable, have too generally in practice proved to be so. There are not wanting, however, many persons who ascribe

126 OF THE FUNCTIONS OF THE LUNGS.

these mournful results, not so much to the abstract difficulty of the case, as to imperfect and erroneous views of the functions and relations of these important organs; and who entertain the opinion that the investigation of the subject has been either from preconceived notions, from a too limited view of the phenomena, or from some other cause, so infelicitously conducted, that the conclusions arrived at, have been either merely assumptions, extremely doubtful, or absolutely erroneous.

It is sufficiently obvious that if we are ignorant of the use any part of a machine, it must be the most unlikely thing in the world that we should know how to set about repairing it when it is out of order; and the matter must be still worse, if we should happen to ascribe a use to it which is different or contrary to that which it really fulfils. So, in an animal, if we are ignorant of the use and relations of any organ, it is very improbable that we can understand the nature of its disorders, or treat them in any case successfully, except by the merest accident, which, though it may waken us up to a sense

OF THE LUNGS AND SKIN.

127

of our ignorance, leaves us so blind to the causes of our success that we have no power of repeating it.

Now this is pretty much the actual state of affairs in respect to diseases of the lungs. No investigation of any organ is worth anything, unless it include its relations with other organs in the same machine.

What should we ever learn by looking at the mainspring of a watch, apart from the general machinery to which it belongs? Though we should look for ever, and employ a microscope to boot, it is clear we should never arrive at the perception of its true relations.

Abernethy's inquiry derived great interest from the investigation of the skin by which it was preceded, and which seems to have formed his primary object. A few words on this wonderful organ may help the unprofessional reader to form some estimate of its relations and importance. As in all animals, it is the surface in immediate contact with external influences-the first which attracts our