220 Prof. Tyndall on the History of Calorescence. 



of the lamp was lodged in the lens in front of it, and in the 

 sides of the cell containing my solution of iodine ; and I deter- 

 mined, as soon as I could turn my thoughts to it, to supplant 

 that lens and that cell by rock-salt ones. 



At that time my experiments on the action of aqueous vapour 

 as an absorbent of radiant heat were at variance with those of 

 an eminent natural philosopher. Respect for him, and the de- 

 sire to place my results beyond the pale of doubt, determined 

 my course of action. The autumn of 1862 was devoted to the 

 preparation of a memoir " On the Relation of Radiant Heat to 

 Aqueous Vapour/'' which was presented to the Royal Society on the 

 20th of November, and read on the 18th of December of that 

 year. In the autumn of 1863 I completed my demonstration of 

 the action of aqueous vapour on radiant heat, and at the same 

 time executed further experiments on dissolved iodine, illustra- 

 tive of its extraordinary transparency to heat-rays of ultra-red 

 refrangibility. 



My course was now cleared, and my thoughts reverted at once 

 to the experiments which I had suspended more than two years 

 before. I went to the Isle of Wight in March 1864 with my 

 ideas perfectly defined and ripe for execution. While there, 

 under circumstances which I have fully described in my article 

 " On Negative Fluorescence/-' published in the January Number 

 of the Philosophical Magazine, I received a note from Dr. Akin, 

 a Hungarian gentleman who had arrived in this country in the 

 early part of 1862, and who was personally unknown to me. 

 His name had been mentioned to me by Professor Stokes ; I had 

 read an article from his pen published in the { Saturday Review/ 

 and he now forwarded to me a proof of a Report presented by 

 him to the British Association assembled at Newcastle in 1863. 

 In this Report three experiments were proposed : two of them 

 were impracticable, but they nevertheless showed ability, while 

 the third seemed to offer a promise of real success. These 

 were not the experiments that I had proposed to make ; I did not 

 intend to experiment on the sun, for his beams are unattainable 

 where I work ; I intended to operate with the radiant source 

 with which the constant practice of the previous eleven years 

 had rendered me familiar, and to realize ideas of which the origin 

 has been indicated above, but which, with the caution of a man 

 who knows the difference between conception and performance, 

 I did not publish in detail. The reader will now be in a posi- 

 tion to appreciate the use which Dr. Akin has made of the fol- 

 lowing note written to him at the time : — 



" Clarendon Hotel, Chale, Isle of Wight, 

 Wednesday. 

 " My dear Sir, — I have read the proof which you have been kind 

 enough to send me with extreme interest. Being a truant from the 



