Faraday to Jacob Herbert   9 March 1847

Royal Institution | 9 March 1847.

My dear Sir

In reply to your letter of the 11th February¹ respecting the Rape oil Argand lamp of Mr. George Herbert I now write to say that I have compared that lamp with the Ordinary Trinity Sperm oil Argand lamp which you sent me using also the specimens of Rape & sperm oils which I received from you. I have burned the lamps continuously for several days & the results of my trials are as follows. The Rape oil lamp has equal horizontal divergence with the sperm oil lamp. It burns with equal constancy; & when the combustion is carried on for long periods as 7 hours or more perhaps with more constancy i.e. it may go longer without the cotton requiring to be retrimmed. The light is not quite so white as that of the Sperm oil light which is probably dependant on the well known greater inflammability of sperm oil. I endeavoured to burn the lamps with equal lights and in obtaining this effect I consumed a little more of the Rape oil than of the sperm oil. If this be confirmed by the results of many lamps burning month after month it may probably be referred also to the more ready combustibility of sperm oil which makes it burn more completely - but the difference after all was not great being as 56 sperm oil to 61 Rape oil.

I am about to proceed with the experiments; introducing the central deflector into the Rape oil lamp according to your letter of the 4th March² & when they are completed will send you the further result. It is to be understood that the comparison in this letter holds between the Sperm oil lamp & the Rape oil lamp in its first or simple state i.e. in the same condition as the sperm oil lamp³[.]

I am | My dear Sir | Your Very faithful Servant | M. Faraday

Jacob Herbert Esq | &c &c &c