Faraday to Jacob Herbert   21 October 1840

Report of Professor Faraday, on the opaque appearance of certain cylinders brought from the North Foreland Light House, - dated October 1840.

Sir,

I yesterday received from Captain Welbank¹ two lamp glasses, on the inner surface of which a thin white film had been produced by the action of the heat of the lamp flame and which by destroying the necessary transparency of the glass is very injurious to the effect of the lamp.

On examination I find this substance to be derived from the glass itself by the action of the heat, and not to be a deposit on it from the Oil of the flame or the current of air. Though in small quantities I was able to determine that it consisted of

Silica

Oxide of lead, and

Alkali in the state of sulphate

and as these are constituents of the glass and can, under the circumstances, be derived from no other source, they shew that the glass itself has been affected by the heat.-

All glass when heated to a sufficient degree and for a sufficient time becomes converted into an opaque crystalline substance looking like anything rather than glass; I send for inspection a valuable specimen of a change of this kind. The glass of which the two lamp glasses are formed is of the kind called flint glass and softens very soon by heat; so that it is not at all surprizing, that the heat of the lamp flame, if continued long enough, should produce the change observed. This leads to the thought that glass which requires a higher heat for it’s fusion, would perhaps resist the change in question. Now such glass as for instance plate and crown glass are, usually, not of so good a colour as flint glass; nor can they be worked so easily because the requisite temperature is higher. But some time since the Excise placed before me specimens of glass from Newcastle which I think would answer for lamp glasses in other respects if, as may be hoped, they would withstand, as can only be ascertained by trial, the present particular action of heat. The glass I refer to was of the same materials as crown glass but these were so well selected that the colour was as good as that of plate glass and even of much Flint glass. The manufacturer worked it into medicine phials and other thin forms and the trade, or some members of it petitioned that it might be charged duty as flint glass so good was it’s appearance and quality. If thought desirable I could ascertain the name of the manufacturer from the Authorities of the Excise Office; but probably other manufacturers could, if required, produce the same result².

I have the honor to be | Sir | Your obedient humble Servant | (signed) M. Faraday

Royal Institution | 21st October 1840.

To | Jacob Herbert, Esq. | Secretary | &c. &c. &c. | Trinity House.