Phil.—­I have heard so many and such various opinions on the nature of the function of respiration during my education and since, that I should like to know what is the modern doctrine on this subject.  I can hardly refer to better authority than yourself, and I have an additional reason for wishing for some accurate knowledge on this matter, having, as you well know, been the subject of an experiment in relation to it which, but for your kind and active assistance, must have terminated fatally.

The Unknown.—­I shall gladly state what I know, which is very little.  In physics and in chemistry, the science of dead matter, we possess many facts and a few principles or laws; but whenever the functions of life are considered, though the facts are numerous, yet there is, as yet, scarcely any approach to general laws, and we must usually end where we begin by confessing our entire ignorance.

Eub.—­I will not allow this ignorance to be entire.  Something, undoubtedly, has been gained by the knowledge of the circulation of the blood and its aeration in the lungs—­these, if not laws, are at least fundamental principles.

The Unknown.—­I speak only of the functions in their connection with life.  We are still ignorant of the source of animal heat, though half a century ago the chemists thought they had proved it was owing to a sort of combustion of the carbon of the blood.

Phil.—­As we return to our inn I hope you will both be so good as give me your views of the nature of this function, so important to all living things; tell me what you know, or what you believe, or what others imagine they know.

The Unknown.—­The powers of the organic system depend upon a continued state of change.  The waste of the body produced in muscular action, perspiration, and various secretions, is made up for by the constant supply of nutritive matter to the blood by the absorbents, and by the action of the heart the blood is preserved in perpetual motion through every part of the body.  In the lungs, or bronchia, the venous blood is exposed to the influence of air and undergoes a remarkable change, being converted into arterial blood.  The obvious chemical alteration of the air is sufficiently simple in this process:  a certain quantity of carbon only is added to it, and it receives an addition of heat or vapour; the volumes of elastic fluid inspired and expired (making allowance for change of temperature) are the same, and if ponderable agents only were to be regarded it would appear as if the only use of respiration were to free the blood from a certain quantity of carbonaceous matter.  But it is probable that this is only a secondary object, and that the change produced by respiration upon the blood is of a much more important kind.  Oxygen, in its elastic state, has properties which are very characteristic:  it gives out light by compression, which is not certainly known to be