218 Prof. Tyndall on the History of Calorescence. 



refer to Lagrange, Mecanique Analytique, 3 e edit, par M. Ber- 

 trand, vol. i. pp. 215, 217, or to Montucla, Histoire des Mathe- 

 matiqueSj vol. iii. p. 618, and more particularly vol. iii. p. 622. 

 Giessen, January 28, 1865. 



XXXII. On the History of Calorescence. 

 By John Tyndall, F.R.S., $c* 



ON the 26th of May, 1859, I presented to the Royal Society 

 a " Note on the Transmission of Radiant Heat through 

 Gaseous Bodies." The question had occupied me some time ; but 

 as the experimental difficulties were very great, I published the 

 Note referred to in order to enable myself to vanquish those dif- 

 ficulties at my leisure. All the time at my disposal in 1859 and 

 1860 was devoted to the subject, and towards the end of 1860 

 I was so far advanced as to be able to prepare a memoir, which 

 was presented to the Royal Society on the 10th of January, and, 

 being chosen as the Bakerian Lecture for that year, was read 

 on the 7th of February, 1861. 



In that memoir the comparative deportment of elementary 

 and compound gases towards radiant heat is for the first time 

 announced. I had attempted to make radiant heat an explorer 

 of molecular condition, and had found that the simple gases 

 possessed a power of transmission immensely greater than that 

 of the compound ones. In the autumn of 1861 I pursued 

 the subject, and by purifying more perfectly the elementary 

 gases, rendered the differences between them and the compound 

 ones still more vast. I then turned my attention to solids and 

 liquids, and confirmed Melloni's experiments on the diather- 

 mancy of lampblack ; I also tried to render the substance more 

 transparent to invisible heat-rays by ridding it of the hydro- 

 carbons which attach themselves to it during its formation. 

 I next examined the element bromine and found it eminently 

 diathermic; I tried sulphur dissolved in bisulphide of carbon 

 and found it still more so. I finally operated on a solution of 

 iodine in bisulphide of carbon, and found that a layer of it, 

 sufficiently dense to intercept completely the light of the noon- 

 day sun, offered a scarcely sensible obstacle to the passage of 

 the invisible calorific raysf 



* Communicated by the Author. 



f The same a priori considerations which led to the discovery of the 

 iodine, point also to red glass coloured by the element gold, instead of that 

 coloured by the suboxide of copper, as most suitable for experiments on 

 ray-transmutation. The colouring matter of the former appears to be 

 without sensible action upon the invisible heat-rays ; and the rays trans- 

 mitted through it are competent to raise platinized platinum to a white heat. 



