Prof. Tyndall on Recent Researches on Radiant Heat. 253 



and vapours on radiant heat was a frequent subject of conversa- 

 tion between my scientific friends and myself; and some of these 

 still remember my remarks at the time ; the hopes I entertained 

 regarding the subject, and the devices by which I proposed to 

 meet its difficulties. I was, however, prevented by other engage- 

 ments from attacking the subject at this time; and not till the 

 early spring of 1859 were my ideas brought to practical defini- 

 tion. Then, however, I devised and applied the apparatus 

 which, with some modifications and improvements, I have used 

 ever since. 



This apparatus immediately opened to me a large and rich 

 field of experimental inquiry ; and the greatest pleasure this dis- 

 covery gave me, and which I often expressed to Mr. Faraday at 

 the time, was, that it placed me in possession of a subject in the 

 prosecution of which I could not possibly interfere with the claims 

 of any previous investigator. The first notice of my researches is 

 published in the ' Proceedings of the Royal Society ' for May 26, 

 1859. On the 10th of June following, I made the investigation 

 the subject of a Friday evening discourse at the Royal Institution. 

 The late lamented Prince Consort was present on this occasion, 

 and with characteristic goodness interested himself afterwards to 

 obtain plates of rock-salt for me. I then executed many of my 

 experiments in presence of a large audience ; and an account of 

 the discourse is published in the ' Proceedings of the Royal 

 Institution ' of the date referred to. I also communicated an 

 account of the investigation to my friend Professor De la Rive, 

 and he published a translation of my communication in the 

 Bibliotheque Universelle. The investigation was also described 

 in Cosmos, in the Nuovo Cimento, and in other Journals. When 

 I reached Paris in 1859, I found that the subject had attracted 

 a greater degree of attention than I could have hoped to see 

 bestowed upon it. In short, the publicity of my mode of experi- 

 ment and results was quite general. 



I will here ask permission to cite a number of these results 

 obtained during the month of July 1859, after the main difficul- 

 ties of my apparatus had been surmounted. The method employed 

 was substantially the same as that described in my last memoir*. 

 The heat passed from the radiating surface through a vacuum 

 into the experimental tube; the principle of compensation was 

 also employed ; the length of the tube used to receive the gas 

 was 12 inches ; and from the galvanometric deflection consequent 

 on the admittance of the gas or vapour its absorption was deduced. 



* Phil. Trans., February 1861 ; Phil. Mag., Sept. 1861. 



