156 Professor Tyndall, [June 10, 



colours were analogous to notes of different pitch. But beyond the 

 visible spectrum in both directions there were rays which excited no 

 impression of light. Those at the red end excited heat, and the 

 reason why they failed to excite light probably was that they never 

 reached the retina at all. This followed from the experiments of 

 Briicke and Knoblauch. These obscure rays had been discovered by Sir 

 \Vm. Herschel, and the speaker demonstrated their existence by placing 

 a thenno-electric pile near to the red end of the spectrum, but still 

 outside of it. The needle of a large galvanometer connected with the 

 pile was deflected and came to rest in a position about 45 degrees 

 from zero. A glass cell, containing the transparent vitreous humour 

 of the eye of an ox, was now placed in the path of the rays : the light oi 

 the spectrum was not perceptibly diminished, but the needle of the 

 galvanometer fell to zero, thus proving that the obscure rays of the 

 spectrum, to which the galvauometric deflection was due, were wholly 

 absorbed by the humours of the eye. 



Reference was made to the excellent researches of Melloni. In a 

 simple and ingenious manner he had proved the law of inverse squares 

 to be true of radiant heat passing through air, and the eminent 

 Italian inferred from his experiments that for a distance of 18 or 20 

 ieei, the action of air upon radiant heat was totally inappreciable. 

 This is the only experimental result now known regarding the trans- 

 mission of radiant heat from terrestrial sources through air ; with 

 regard to its transmission through other gases it was believed that 

 we were without any information. 



It was, however, very desirable to examine the action of such 

 media — desirable on purely scientific grounds, and also on account of 

 certain speculations which had been based upon the supposed deportment 

 of the atmosphere as regards radiant heat. These speculations were 

 originated by Fourier ; but it was to M. Fouillet's celebrated Memoir, 

 and the recent excellent paper of Mr. Hopkins, to which we were 

 indebted for their chief development. It was supposed that the rays 

 from the sun and fixed stars could reach the earth through the 

 atmosphere more easily than the rays emanating from the earth could 

 get back into space. This view required experimental verification, and 

 the more so, as the only experiment we possessed was the negative 

 one of Melloni, to which reference has been already made. 



The energetic action of the solid and liquid compounds into which 

 the element hydrogen enters, suggested the thought that hydrogen gas 

 might act more powerfully than air, and the following means were 

 devised to test this idea. A tube was constructed, having its ends 

 stopped air-tight by polished 'plates of rock-salt held between suitable 

 washers, which salt is known to be transparent to heat of all kinds ; the 

 tube could be attached to an air-pump and exhausted, and any 

 required gas or vapour could be admitted into it. A thermo-electric 

 pile being placed at one end of the tube, and a source of heat at the 

 other, the needle of an extremely sensitive galvanometer connected 

 with the pile was deflected. After it had come to rest, the air was 

 pumped from the tube, and the needle was carefully observed to see 



