The discovery of oxygen gas by Priestley, not only gave a great impetus to chemical inquiries, but affected Physiology in a very remarkable manner; when it was found that the _more obvious_ phenomena of all cases of ordinary burning—lamps, candles, and fires of every kind—consisted mainly of the chemical union of charcoal and oxygen (carbonic acid); and again, when it was discovered that animals, in breathing, somehow or other produced a similar change, one may conceive how ready every one was to cry, "I have found it. The heat of animals is nothing more than combustion! We inhale oxygen; we breathe out carbonic acid; the 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, however, shows that the idea that respiration is the cause of animal heat, or that it is due to any other change of oxygen merely, 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 the oxygen inspired with the charcoal expired; secondly, nothing is so obvious 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.