[The following letters from Lord Kelvin came to light in arranging the scientific correspondence of Sir George Stokes. On supplying their dates to Lord Kelvin, he was able to extend the record. The parts relating to spectrum analysis are here printed, with Lord Kelvin's permission, as supplementary to the extracts contained in pp. 127—136 of this volume: cf. p. 136. It may be recalled that Prof. Stokes became Secretary of the Royal Society in 1854.]

2 College, Glasgow

Feb. 20, 1854.

My dear Stokes

It is a long long time since I have either seen you or heard from you, and I want you to write to me about yourself and what you have been doing since ever so long. Have you made any more revolutions in Science? or done any of the exp research on the friction of air? I saw a notice of your lecture at the R. I. Tell me any new discoveries you have made, &c. However I do not mean to impose upon you by demanding all this, but if there is anything short and good you can tell me I shall be glad to hear it. I want to ask you about artif lights and the solar dark lines. Is there any other substance than soda that is related to D)1 Are bright lines corresponding to it to be seen where soda is not present?