Faraday to Edward Magrath   14 February 1831

Royal Institution, Feb. 14. 1831.

Dear Sir,

The following are the best answers I am able to give to your Queries¹:

1. In an experiment made at the Athenaeum, with an excellent argand oil lamp, regulated by Mr. Hancock ², and compared with a 15-hole gas burner, the light of the gas was to that of the oil as 21 to 13.

2. In experiments made to determine the heat evolved for equal quantities of light from oil and gas burning brightly from argand burners, the heat evolved from the oil being 2, that from the gas was nearly 3.

3. A little sulphurous or sulphuric acid is generally formed from the combustion of coal gas. If well-purified gas be used, this product is rarely sensible; it is less sensible as sulphuric than as sulphurous acid. Upon closely questioning persons who have declared they smelt the sulphur from gas, I have usually found they meant something else; generally the oppressive heat, or the dry sensation, or the smell of a little gas unburnt, none of which have anything to do with the sulphur product from gas.

4. I do not believe that any gas escapes unburnt from a gas light well regulated. It is far more likely that oil vapour should escape unconsumed from an oil burner than gas from a gas burner.

5. Neither oil nor gas ought to soil, or will soil, the ceiling of a room, if well regulated. Either will do so when badly regulated. I think, of the two, oil is the most liable to do so; because of the changes which take place in the wick, in the temperature of the oil, &c., during burning, and which do not occur with a gas lamp properly regulated by a governor.

6. This effect depends upon so many circumstances, as the size and tightness of the room, the proportion of light, &c. &c., that it can only be deduced from a series of observations.

7. It is exceedingly difficult to ascertain, and if ascertained, to describe correctly the effects of lights on air, so as to convey a just opinion of their influence; thus, with regard to their greatest effect, which is the power of heating, it is of advantage and desirable up to a certain point, and above that point is unpleasant and disagreeable: but that point depends upon many other things as well as the lights, and, what is still more important, differs for different persons, so that it becomes in that respect impossible to please all. Gas light will heat air faster than oil light; it therefore at first does good quicker, and afterwards does harm quicker than oil. As to the proportionate deterioration of air by the oxygen abstracted, I think it probable that gas would, light for light, have the greatest effect: but I do not believe that effect would be sensible in either case. As to the deterioration of air by the sulphurous acid and unburnt gas thrown into the room, I think little of it in the present case. I think that to be rather a popular error, caused by persons ascribing effects which they feel, but cannot discriminate, to the first cause which occurs to them, thinking of quality only, and forgetting quantity.

8. When the oil was burning in its best manner, still it gave a much yellower flame that the gas; the whiteness of the gas flame is a necessary consequence of its higher temperature.

9. In my opinion the principal cause of complaint is of the following nature: A house has been built, and every endeavour made to render floors, ceilings, windows, walls, and doors, tight and close; the rooms in it are well warmed during the day, and, having been brought to such a temperature and state that the first person who enters is fully satisfied, from fifty to two hundred persons are introduced, evolving both heat and effluvia; a number of powerful burners are put into and continue in action, and when the injurious agency of these causes has continued for one, two, or three hours, complaints are made that the heat is oppressive, or the odour unpleasant. Things are arranged so as to produce a perfect effect under one set of circumstances, and then, changing the circumstances, the effect is expected to remain the same, though it must of necessity be different. The large room and the library are made quite warm enough by daylight, when there are only a few persons there: then they are lighted, many persons enter, and they must of course very soon expect an oppressive sensation. I have no hesitation in believing that the cause of these complaints might be removed by extending and adjusting the system of ventilation in those rooms.

10. Oil has been displaced by oil gas, in the first place because of the economy of the latter; then because of its superior cleanliness, and its facile management. Much harm was done to our seats in the lecture-room during the use of oil lamps. We still burn oil in table lamps in our library, on other evenings than the Fridays, because we require less light on those occasions.

I am, dear Sir, | Very truly yours, | M. FARADAY.

E. Magrath, Esq.