Slough Sep. 13. 1825. | abstract
M. Faraday Esq. RI.
Dear Sir,
I am most obliged to you for your copy of your Paper with the Errata1. I will take care that they shall be inserted in the end of the volume2.
I received some time ago a packet of specimens of Glass of Exp. 1. with a paper of determinations of the Specific gravities of different portions & from different situations in the pots. This I presume was from you though with no name. The difference of S.G between the top & bottom in one case exceeds anything I could have supposed possible. It is evident that no accidental defect in mixing it could have produced it. A separation, by subsidence, of one fluid within the other, has evidently taken place. This is very remarkable, and indicates that we ought to aim at making atomic compounds, or at least compounds capable of permanent mixture[.] I therefore3 formed an atomic silicate of Lead (S̈ L̇). Its refractive index came out so high as 2.123 for Extreme Red rays & therefore probably as high as 2.2 for mean, which approaches to the refraction of Phosphorus and exceeds glass of Antimony. Its dispersion is Enormous - so much so that I could not measure it with my usual apparatus. A prism of 21° 12’ required to be opposed by three prisms of flint glass of 30° to neutralize the colour[.] This glass however is too soft, and cannot be cooled in large masses quick enough to prevent crystallizing & losing its transparency - but, what is very singular is that I could not obtain it otherwise than full of steaks & veins though its fusion is as liquid as water & it was well agitated before[.]
The following passage is crossed through here: “On comparing the dispersive powers of the glass in these three pots and those analyzed at the R. Institution before the commencement of the experiments, with a series of dispersive powers of 5 kinds of Flint-glass of which I have given an account in the Edinb. Trans. Vol. X[I] [Herschel (1822), 459]. I am surprised to find that while the refractive indices differ very little, the dispersions of the former glasses are much greater[.] The mean of the Refractive indices of 9 specimens now examined is 1.60[.] The mean of their dispersive indices 0.081[.] The mean of the Refractive indices of 5 specimens formerly examined in 1.59 that of their dispersive indices 0.064.
The quantity of Lead therefore exerts a much greater influence on the dispersive than on the refractive power.
Yet this is true only within certain limits. I have”.
FARADAY, Michael (1825b): “On new compounds of carbon and hydrogen, and on certain other products obtained during the decomposition of oil by heat”, Phil. Trans., 115: 440-66.
HERSCHEL, John Frederick William (1822): “On the Absorption of Light by Coloured Media, and on the Colours of the Prismatic Spectrum exhibited by certain Flames; with an Account of a ready Mode of determining the absolute dispersive power of any Medium by direct experiment”, Trans. Roy. Soc. Edinb., 9: 445-60.
Please cite as “Faraday0267,” in Ɛpsilon: The Michael Faraday Collection accessed on 27 April 2024, https://epsilon.ac.uk/view/faraday/letters/Faraday0267