240 TEMPERATURE OF SPACE. SECT. XXVI. 



The climate of Venus more nearly resembles that of 

 the earth, though, excepting perhaps at her poles, much 

 too hot for animal and vegetable life as they exist here ; 

 but in Mercury, the mean heat arising only from the 

 intensity of the sun's rays must be above that of boiling 

 quicksilver, and water would boil even at his poles. 

 Thus the planets, though kindred with the earth in mo- 

 tion and structure, are totally unfit for the habitation of 

 such a being as man, unless, indeed, their temperature 

 should be modified by circumstances of which we are 

 not aware, and which may increase or diminish the 

 sensible heat so as to render them habitable. 



It is found by experience, that heat is developed in 

 opaque and translucent substances by their absorption of 

 solar light, but that the sun's rays do not sensibly alter 

 the temperature of perfectly transparent bodies through 

 which they pass. As the temperature of the pellucid 

 planetary space can be but little affected by the passage 

 of the sun's light and heat, neither can it be sensibly 

 raised by die heat now radiated from the earth ; conse- 

 quently its temperature must be invariable, at least 

 throughout the extent of the solar system. The at- 

 mosphere, on the contrary, gradually increasing in den- 

 sity toward the surface of the earth, becomes less pel- 

 lucid, and therefore gradually increases in temperature, 

 both from the direct action of the sun, and from the ra- 

 diation of the earth. Lambert had proved that the ca- 

 pacity of the atmosphere for heat varies according to the 

 same law with its capacity for absorbing a ray of light 

 passing through it from the zenith, whence M. Svanberg 

 found that the temperature of space is 58 below the 

 zero point of Fahrenheit's thermometer. From other 

 researches, founded upon the rate and quantity of at- 

 mospheric refraction, he obtained a result which only 

 differs from the preceding by half a degree. M. Fourier 

 has arrived at nearly the same conclusion from the law 

 of the radiation of the heat of the terrestrial spheroid, 

 on the hypothesis of its having nearly attained its limit 

 of temperature in cooling down from its supposed prim- 

 itive state of fusion. The difference in the result of 

 these three methods, totally independent of one another, 

 only amounts to the fraction of a degree. 



