Faraday to Thomas Byam Martin   7 August 1854

[Royal Institution embossed letterhead] | Royal Institution | 7 Aug 1854

My dear Sir Byam

I send you herewith the best answer I can make to your enquiries¹[.]

Ever Your faithful Obedient Servant | M. Faraday

Observations &c.

Royal Institution 7 August 1854.

Very few of the questions are so put that I, in reference to their chemical or physical character, can give any consistent or distinct answer to them. The proposition is correct in theory, i.e. dense smoke will hide objects, and burning sulphur will yield fumes that are intolerable, and able to render men involved in them incapable of action, or even to kill them: but whether the proposition is practicable on the scale proposed and required, is a point so little illustrated by any experience, or by any facts that can be made to bear upon it, that for my own part I am unable to form a judgment. I have been on the crater of Vesuvius² and to leeward of the mouth; and have seen the vapours (which are very deleterious) pass up over my head and go off down the wind in a long and not rapidly expanding stream. I have, by changes in the wind, been involved in the vapours, and have managed with a handkerchief to the mouth and by running, to get out of their way. I should hesitate in concluding that ten or twenty vessels could give a body of smoke, the columns of which, at a mile to leeward, would coincide and form an impervious band to vision a mile broad; but I have no means of judging, for I know of no sufficient facts that can be of use as illustrations of the proposed applications.

In reference to the burning of sulphur and formation of sulphurous acid, I may remark, that, as 400 tons of sulphur have been spoken of, perhaps the following considerations may help to give some general ideas, in the present state of the proposition, as to the probable effect of its fumes. If a ship charged with sulphur were burning in a current of air, a continuous stream of sulphurous acid fumes, mingled with air, would pass off from it. This stream, being heavier than air, would descend and move along over the surface of the water; and, I expect, would sink perpendicularly and expand laterally, so as to form a low broad stream. Its noxious height would probably soon be less than 15, or perhaps even 10, feet, (but I cannot pretend to more than a guess) and its width by degrees more and more. The water over which it would have to move, would tend continually to take part of the noxious vapour out of it. Now 400 tons of sulphur would require 400 tons of oxygen; and that it would find in about 1740 tons of air. Supposing that this product were mixed with ten times its bulk of unaltered air, it would make near upon 20000 tons of a very bad mixture; and one, which if a man were immersed in it for a short time, would cause death. Supposing that the 20000 tons of mixed deleterious air were converted into a regular stream, 30 feet high and 300 feet wide, then it would be about 6500 feet or a mile & a quarter long. Such is a representative result for 400 tons of sulphur, and hence an idea may be formed of the time during which with a given velocity of wind, the places involved in the stream may remain subject to its effects.

In respect of the seven questions³, there is scarcely a point in them to which I am able to give an answer of any value.

As to 1⁴.; I suspect much larger quantities of matter will be required than is supposed.- I do not imagine that if burnt in heaps coals would burn fast enough to give the smoke required.

2⁵. The data are wanting.

3⁶. I suspect the upper part of high buildings would frequently be free from the sulphurous vapours; and that sets of eddies of fresh air from above would occur behind.

4⁷

5⁸

6⁹ The lateral extent at the distance of a mile very doubtful - would need proof[.]

7¹⁰.

The proposition is, as I have said, correct in theory, but in its results must depend entirely on practical points. These are so untried and unknown, and there are so few general facts bearing on the subject, that I have the utmost difficulty in speaking at all to the matter. These circumstances must plead my excuse for the very meagre character of the only observations which I am at present able to offer. All I need add is, that if the project were known or anticipated, it would not be difficult for the attacked party to provide respirators, which would enable the men, in a very great degree or even altogether, to resist a temporary invasion of an atmosphere such as that described.

M. Faraday

Endorsement: Paper from Professor Faraday in answer to questions by the comtt¹¹.