J46 THE LIFE OF JAMES D. FORBES. CHAP. 



friend Ellis's success, as well as in tie honour of Trinity. 

 I fear I shall scarcely know Cambridge soon, if my old 

 friends take wing as they have lately been doing. Gregory 

 gives me the hope that if I go to Cambridge in May, 

 which I have some thoughts of doing, I shall find you 

 lecturing on Moral Philosophy/ 



To the Same. 



'EDINBURGH, February 22nd, 1840. 



' You cannot gratify me more than by leading me to 

 imagine, what I wish I could persuade myself of, that 

 I can ever render you any service worth giving. . . . 

 I look forward with much interest to the appearance of 

 your book, and on several accounts. Some views you 

 once stated to me as we walked down the north bank of 

 the Cam, some two years ago, have been sticking by me 

 since, and I expect to see them developed. Will it be 

 one volume ? If so, it must be nearly complete. 



' I have not quite, but nearly done tormenting heat 

 with gratings and dusty diaphragms. All I can do, all 

 kinds of heat get through the finest gratings (metallic) 

 in precisely equal proportions, and that equal to the area 

 of interstices ; and yet grooved surfaces exert a powerful 

 specific action. I am reluctantly forced to the conclusion 

 too, that pure metals may be reduced to powder so fine 

 as to affect the quality of the heat transmitted, though 

 the thinnest gold leaf permits no appreciable portion of 

 heat to pass/ 



To SIR J. F. W. HERSCHEL, BART. 



'THE DEAN HOUSE, EDINBURGH, March 9^, 1840. 



' . . . I should like very much to know the degree of 

 sensibility of your paper to heat ; whether, for instance, 

 the heat of the hand affects it. It has long been an 

 object with me to get a surface capable of detecting heat 

 pictures, such as those which polarization and diffraction 

 would indicate. Mr. Talbot gave me hopes at Birming- 

 ham of providing me with such, and he actually sent me 

 a primrose- coloured paper which was transiently affected 



