To His Excellency Benjamin Franklin December, 31 1785
From the original letter at the American Philosophical Society.
Dec. 31st 1785
I send you the Candles I have been making. In a little time after they are lighted the smoke and flame separate, the one issuing from one end of the candle, and the other from the other end. I suppose this to be because a quantity of Air enters into the candle between the Tallow and the flame, and in its passage downwards takes the smoke with it, for if you blow a quantity of air up the Candle, the current will be changed, and the Smoke re-ascends and in passing this the flame makes a small flash and a little noise.
But to express the Idea I mean, of the smoke descending more clearly it is this, — that the Air enters the candle in the very place where the melted tallow is getting into the state of flame, and takes it down before the change is completed — for there appears to me to be two kinds of smoke, humid matter which never can be flame, — and inflammable matter which would be flame if some accident did not prevent the change being completed — and this I suppose to be the case with the descending smoke of the Candle.
As you can compare the Candle with the Lamp, you will have an opportunity of ascertaining the cause — why it will do in the one and not in the other — When the edge of the inflamed part of the wick is close with the edge of the Tin of the Lamp no counter current of air can enter — but as this contact does not take place in the candle a counter current can enter and prevents the effect in the candles, which illuminates the Lamp — for the passing of the Air through the Lamp does not, I imagine, burn the smoke, but burns up all the oil into flame, or by its rapidity prevents any part of the oil flying off in the state of half-flame which is smoke —
I do not my dear Sir, offer these reasons to you but to myself, for I have often observed that by lending words for my thoughts I understand my thoughts the better — Thoughts are a kind of mental smoke, which require words to illuminate them.
I am affectionately your obt hble servant
I hope to be well enough tomorrow to wait on you.