154 ANNUAL OF SCIENTIFIC DISCOVERY. 



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. This heat is entirely ab- 

 sorbed in our atmosphere ; and on the evening in question it was in 

 part applied to evaporate the precipitated particles, hence to aug- 

 ment 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, I may remark, was furnished with a conical reflec- 

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

 itself. 



In a paper recently communicated to the Royal Society, Professor 

 Tyndall has detailed the results of some additional investigations 

 on the absorption of heat by gases and vapors. The absorption of 

 radiant heat by atmospheric air in a short tube, and at a tension of 

 thirty inches, being taken as 1 ; chlorine would be 36 ; hydrochloric 

 acid, 62; carbonic acid, 90; sulphuretted hydrogen, 390; olefiant 

 gas, 970; ammonia, 1,195. The absorption of radiant heat by vapors 

 was also found to be very considerable, and even small quantities of 

 perfumes, when diffused through common air, increase its power of 

 arresting heat to an extraordinary degree : thus the absorptive power 

 of air charged with the perfume of patchouli is thirty times greater 

 than that of pure air ; lavender increases the power to sixty times, 

 and aniseed three hundred and seventy-two times the natural amount ; 

 hence the perfume arising from a bed of flowers increases the tem- 

 perature of the air around them by rendering it more absorptive of 

 radiant solar heat. The vapor of water, when present, increases the 

 absorption of heat by the air in a very extraordinary degree. Dr. 

 Tyndall infers that as the amount of vapor diminishes rapidly at a 

 distance from the earth's surface, the sun's rays are not sensibly ar- 

 rested until they reach our atmosphere, but that, on the other hand, 

 the heat of the earth is prevented from being radiated into the free 

 space and lost, by the existence of vapor in the air ; and owing to 

 the influence of this action, he thinks that even those planets most 

 distant from the sun may have a temperature sufficiently high to 

 render them inhabitable. 



Radiation of Heat at Night. About the period of sunset, pro- 

 vided the sky be clear, the temperature of the air in contact with the 

 earth's surface is cooler than that of the atmosphere at a certain 

 height above the ground. This is attributable to the gradual cooling 

 of the earth's surface, arising; from the nocturnal radiation of the 



* ^J 



heat into empty space. The cooling of the surface of the earth nat- 

 urally gives rise to a corresponding diminution of the temperature of 

 the stratum of air in its immediate vicinity ; the effect is commu- 

 nicated to the stratum above, though naturally in a less degree, and 

 so on from one stratum to another, until a height be attained at 

 which the temperature of the atmosphere is found to be equal to 

 that of the stratum of air in contact with the earth. Prof. Marcet 



densed to a liquid shell enveloping- the earth, the experiments of Mclloni would 

 lead us to conclude that such a shell would completely intercept the obscure ter- 

 restrial rays. And if the vapor be equally energetic, our atmosphere would pre- 

 vent the direct transmission of the obscure heat of the earth into space. On thia 

 point, however, I wish to make some further observations. 



