From John Tyndall   23 February [1871]¹

My dear Darwin.

I have been recently working at Respirators for firemen, and have devised one which permits a man to remain an hour in smoke, a single inhalation of which, without the respirator, would almost choke him.²

I first tried Cotton-wool; but it let the pungent smoke of resinous pine through freely. A friend then suggested the moistening of the wool with Glycerine. Glycerine had been previously used by Pouchet to make particles stick to a plate of glass.³ I afterwards added fragments of charcoal and hair and in this way have obtained an excellent respirator.

I have had firemen here and they say that they could remain for any length of time in the dense smoke that I had prepared for them.

But what I write to you about is this. The glycerine was a great improvement: even without the coal it does much to quell the most intolerable smoke— it intercepts with extraordinary effect all the floating matter of the air.

Now the human nose with its hairs and mucous is a respirator less perfect than mine, it is true, but still to a great extent effectual.

Supposing a savage tribe to be afflicted with epidemic disease which is undoubtedly propagated by floating particles and that some of them had by a good fortune hairs within their noses: suppose them to breathe through the nose, as many savages to. The chance of survival would certainly any be with those possessing the hairs within the noses. Hairy nosed men would thus be left behind after men without this protection had been cut off. And hence we should have the hairs propagated as we now have them.

I believe this question of hairs within the nose has been a difficulty: but it certainly falls under your general principle.

Yours truly | John Tyndall

Perhaps I am writing about what is already known to you.