1894.] on the Scientific Work of Tyndall. 219 



hope now to repeat. He found that it was possible by means of a 

 solution of iodine in bisulphide of carbon to isolate the invisible rays. 

 This solution is opaque to light; even the sun could not be seen 

 through it ; but it is very fairly transparent to the invisible ultra-red 

 radiation. By means of a concave reflector I concentrate the rays 

 from an arc lamp. In their path is inserted the opaque solution, but 

 in the focus of invisible radiation the heat developed is sufficient to 

 cause the inflammation of a piece of gun-cotton. 



Tyndall varied this beautiful experiment in many ways. By 

 raising to incandescence a piece of platinum foil, he illustrated tho 

 transformation of invisible into visible radiation. 



The most important work, however, that we owe to Tyndall in 

 connection with heat is the investigation of the absorption by gaseous 

 bodies of invisible radiation. Melloni had examined the behaviour 

 of solid and liquid bodies, but not of gaseous. He found that trans- 

 parent bodies like glass might be very opaque to invisible radiation. 

 Thus, as we all know, a glass screen will keep off the heat of a fire, 

 while if we wish to protect ourselves from the sun, the glass screen 

 would be useless. On the other hand rock salt freely transmitted 

 invisible radiation. But nothing had been done on the subject of 

 gaseous absorption, when Tyndall attacked this very difficult problem. 

 Some of his results are shown in the accompanying table. The 

 absorption of the ordinary non-condensable, or rather, not easily 

 condensable gases — for we must not talk of non-condensable gases 

 now, least of all in this place — the absorption of these gases is very 

 small ; but when we pass to the more compound gases, such as nitric 

 oxide, we find the absorption much greater — and in the case of 

 defiant gas we see that the absorbing power is as much as 6000 

 times that of the ordinary gases. 



Relative Absorption at 

 1 Inch Pressure. 



Air 1 



Oxygen 1 



Nitrogen 1 



Hydrogen 1 



Carbonic acid 972 



Nitric oxide 1590 



Ammonia 5460 



Olefiautgas 6030 



There is one substance as to which there has been a great diversity 

 of opinion — aqueous vapour. Tyndall found that aqueous vapour 

 exercises a strong power of absorption — strong relatively to that of the 

 air in which it is contained. This is of course a question of great 

 importance, especially in relation to meteorology. Tyndall's conclu- 

 sions were vehemently contested by many of the authorities of the time, 

 among whom was Magnus, the celebrated physicist of Berlin. With 

 a view to this lecture I have gone somewhat carefully into this ques- 

 tion, and I have been greatly impressed by the care and skill showed 

 by Tyndall, even in his earlier experiments upon this subject. He 



Q 2 



