662 THE POPULAR SCIENCE MONTHLY. 



found that transparent 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 will be useless. On the other 

 hand, rock salt freely transmitted invisible radiation. But noth- 

 ing had been done on the subject of gaseous absorption, when 

 Tyndall attacked this very difficult problem. Some of his re- 

 sults 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 

 olefiant gas we see that the absorbing power is as much as 6,000 

 times that of the ordinary gases. 



Relative Absorption at 

 1 inch Pressure. 



Air 1 



Oxygen 1 



Nitrogen 1 



Hydrogen 1 



Carbonic acid 9*72 



Nitric oxide 1,090 



Ammonia 5,460 



Olefiant gas 6,030 



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

 diversity of opinion aqueous vapor. Tyndall found that aque- 

 ous vapor exercises a strong power of absorption strong rela- 

 tively 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 conclusions were vehemently contested 

 by many of the authorities of the time, among whom was Mag- 

 nus, the celebrated physicist of Berlin. With a view to this lec- 

 ture I have gone somewhat carefully into this question, and I 

 have been greatly impressed by the care and skill showed by 

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

 was at once sanguine and skeptical a combination necessary for 

 success in any branch of science. The experimentalist who is not 

 skeptical will be led away on a false tack and accept conclusions 

 which he would find it necessary to reject were he to pursue the 

 matter further ; if not sanguine, he will be discouraged altogether 

 by the difficulties encountered in his earlier efforts, and so arrive 

 at no conclusion at all. One criticism, however, may be made. 

 Tyndall did not at first describe with sufficient detail the method 

 and the precautions which he used. There was a want of that 

 precise information necessary to allow another to follow in his 

 steps. Perhaps this may have been due to his literary instinct. 



