Pièce 27 - James David Forbes to William Whewell

Edinburgh - JDF has not seen the experiments by Biot [Jean Baptiste Biot] on circular polarisation which WW gave an account of in his last letter: they 'must have been made subsequent to, and in consequence of mine'. WW is wrong to suggest to JDF that he has neglected the influence of the crystalline structure on heat: 'Depend upon it that Biot has not got the effects you mention, with Dark heat, and hence the information it gives is only partial, shewing that light and heat disappear together as they always do'. He has been investigating the action of metals on heat which show both remarkable analogies but also differences with light: 'P.S. Since writing the above I have (partly) a doubt which has much perplexed me. The action of metals gives a maximum polarizing angle for heat greater than for Light, whence I concluded that the index of refraction must be greater for the former than the latter, - contrary to my general views'. He finds in David Brewster's paper 'that a precisely similar fact occurs in the action of metals on light; the Red ray is polarized at a greater incidence than the blue'.