It has been shown by some noted French experimenters, that the amount of combustion increases up to about the thirtieth year, remains stationary to about forty-five, and then diminishes. This last is the point where old age starts from. The great fact of physical life is the perpetual commerce with the elements, and the fire is the measure of it. About this time of life, if food is plenty where you live,—for that, you know, regulates matrimony,—you may be expecting to find yourself a grandfather some fine morning; a kind of domestic felicity that gives one a cool shiver of delight to think of, as among the not remotely possible events. I don’t mind much those slipshod lines Dr. Johnson wrote to Thrale, telling her about life’s declining from _thirty-five_; the furnace is in full blast for ten years longer, as I have said. The Romans came very near the mark; their age of enlistment reached from seventeen to forty-six years. What is the use of fighting against the seasons, or the tides, or the movements of the planetary bodies, or this ebb in the wave of life that flows through us? We are old fellows from the moment the fire begins to go out. Let us always behave like gentlemen when we are introduced to new acquaintance. _Incipit Allegoria Senectutis_. Old Age, this is Mr. Professor; Mr. Professor, this is Old Age. _Old Age_.—Mr. Professor, I hope to see you well. I have known you for some time, though I think you did not know me. Shall we walk down the street together? _Professor_ (drawing back a little).—We can talk more quietly, perhaps, in my study. Will you tell me how it is you seem to be acquainted with everybody you are introduced to, though he evidently considers you an entire stranger? _Old Age_.—I make it a rule never to force myself upon a person’s recognition until I have known him at least _five years_. _Professor_.—Do you mean to say that you have known me so long as that? _Old Age_. I do. I left my card on you longer ago than that, but I am afraid you never read it; yet I see you have it with you. _Professor_.—Where? _Old Age_.—There, between your eyebrows,—three straight lines running up and down; all the probate courts know that token,—“Old Age, his mark.” Put your forefinger on the inner end of one eyebrow, and your middle finger on the inner end of the other eyebrow; now separate the fingers, and you will smooth out my sign-manual; that’s the way you used to look before I left my card on you. _Professor_.—What message do people generally send back when you first call on them? _Old Age_.—Not at home. Then I leave a card and go. Next year I call; get the same answer; leave another card. So for five or six,—sometimes ten years or more. At last, if they don’t let me in, I break in through the front door or the windows.