NATURAL PHILOSOPHY. 153 



the boiler be so arranged that there shall be a constant circulation 

 kept up by the difference of temperatures of different parts. A 

 second precaution easily taken is never to close a boiler when at rest, 

 hermetically, but to keep the safety-valve slightly raised, or a steam- 

 cock a little open, so that a small quantity of steam may always be 



forming. 



RADIATION AND ABSOPtPTION. 



The following note, addressed to Sir John Herschel by Professor 

 Tyndall, is published in the London Mechanics' Magazine : I have 

 been for some time experimenting on the permeability of our atmos- 

 phere to radiant heat, and have arrived at the conclusion that true 

 air, that is to say, the mixture of oxygen and nitrogen which forms 

 the body of our atmosphere, is, as regards the transmission of radiant 

 heat, a practical vacuum. The results from which the opacity of air 

 has been inferred are all to be ascribed to the extraneous matters 

 diffused in the atmosphere, and mainly to the aqueous vapor. The 

 negative results recently obtained by that eminent experimenter, 

 Professor Magnus, of Berlin, have induced me to re-investigate this 

 point ; and the experiments which I have made not only establish 

 the action of aqueous vapor, but prove this action to be compara- 

 tively enormous. Here is a typical case: On the 10th of this 

 month, I found the absorptive action of the common air of our labo- 

 ratory to be made up of three components, the first of which, due to 

 the pure air, was represented in magnitude by the number one ; the 

 second, due to the transparent aqueous vapor, was represented by the 

 number forty ; while the third, due to the effluvia of the locality and 

 the carbonic acid of the air, was represented by the number twenty- 

 seven. The total action of its foreign constituents on the day in ques- 

 tion was certainly sixty-seven times that of the atmosphere itself; 

 while the aqueous vapor alone exerted an action at least forty times 

 that of the air. 



I have also to communicate to you some results of lunar radiation 

 which connect themselves with your speculations. On the 18th of 

 October, 1861, I made a series of observations on the moon, from 

 the roof of the Royal Institution. From six concurrent experiments, 

 I was compelled to infer that my thermo-electric pile lost more heat 

 when presented to the moon than when turned to any other portion 

 of the heavens of the same altitude. The effect was equivalent to a 

 radiation of cold from our satellite. I was quite unprepared for this 

 result, which, however, you will at once perceive may be an imme- 

 diate consequence of the moon's heat. On the evening in question, a 

 faint halo which surrounded the moon, and which was only visible 

 when sought for, showed that a small quantity of precipitated vapor 

 was afloat in the atmosphere. Such precipitated particles, in virtue 

 of their multitudinous reflections, constitute a powerful screen to 

 intercept the terrestrial rays, and any agency that removes them and 

 establishes the optical continuity of the atmosphere must assist the 

 transmission of terrestrial heat. 1 I think it may be affirmed that no 



1 I was going to add " into space,' 1 but the expression might lead to misappre- 

 hension. My experiments indicate that the absorption of water is a molecular 

 phenomenon. If we suppose the aqueous vapor of the atmosphere to be con- 



