NATURAL PHILOSOPHY. 159 



due to the pure air being represented in magnitude by No. 1 ; the 

 second due to the transparent aqueous vapor by No. 40 ; and the 

 third due to the effluvia of the locality, and the carbonic acid of the 

 air by No. 27. The total action of its foreign constituents was cer- 

 tainly sixty-seven times that of the atmosphere itself, while the 

 aqueous vapor alone exerted an action at least forty times that of the 

 air. On Oct. 18, Prof. Tyndall made a series of observations on the 

 moon from the roof of the Royal Institution. From six concurrent 

 experiments, he says : " I was compelled to infer that my thermo-elec- 

 tric pile lost more heat when presented to the moon than when turned 

 to any other portions 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 immediate consequence of the moon's heat. On the even- 

 ing in question a faint halo which surrounded the moon, and which 

 was only visible when sought for, showed that a small quantity of pre- 

 cipitated vapor was afloat in the atmosphere. Such precipitated par- 

 ticles, in virtue of their multitudinous reflections, constitute a power- 

 ful screen to intercept the terrestrial rays ; and any agency that 

 removes them and establishes the optical continuity of the atmos- 

 phere must assist the transmission of terrestrial heat. I think it may 

 be affirmed that no sensible quantity of the obscure heat of the moon, 

 which, when she is full, probably constitutes a large proportion of the 

 total heat emitted in the direction of the earth, reaches us. The heat 

 is entirely absorbed in our atmosphere, and on the evening in ques- 

 tion it was in part applied to evaporate the precipitated particles, 

 hence to augment the transparency of the air round the moon, and 

 thus to open a door in that direction for the escape of heat from the 

 face of my pile. The instrument was furnished with a conical reflec- 

 tor, the angular area of which was very many times that of the moon 

 itself." 



CONDUCTION OF HEAT BY GASES. BY G. MAGNUS. 



The cooling of a body in vacua depends simply on the exchange of 

 heat by radiation between the cooling mass and the encircling envel- 

 ope. If the space contains gas, an ascending current is formed, which 

 accelerates the cooling, added to which the property which the gas 

 has of transmitting heat, or its diathermancy, concurs in producing 

 cooling, provided the gases can conduct heat. Dulong and Petit, in 

 enunciating their law of the loss of heat, have neglected the last two 

 actions, manifestly because they are infinitely small compared with 

 the influence of the ascending currents. Since then, it has been uni- 

 versally admitted that the differences in the cooling of the different 

 gases depend on the different mobility of their particles. Cooling 

 takes place much more rapidly in hydrogen than in other gases. 

 With the same amount of heat, this gas expands not more, but less, 

 than atmospheric air; the changes in density in the former gas are 

 less than the latter. But it is the difference of specific gravity which 

 produces currents. If, therefore, different gases by contact with a 

 warmer body all become equally heated, the currents in those gases 

 which have a greater co-efficient of expansion must be greater than in 



