SECT. ii. CHEMICAL POWER OF STAR LIGHT. 55 



is 16,000 times that of dry air, the whole of the heat 

 radiated by the full moon is intercepted by our atmo- 

 sphere. It raises the temperature of the higher re- 

 gions, dissolves the vapour, dissipates the clouds, 

 prevents the formation of more, and allows the heat 

 radiated from the earth to pass freely into space : thus 

 confirming the common, and almost universal, belief that 

 the full moon dispels the clouds. The absorptive power 

 of aqueous vapour is so enormous that even the planet 

 Mercury may be habitable should his atmosphere con- 

 tain a sufficient quantity of it to mitigate the heat of 

 the sun. 



No doubt all the heat from the stars must be ab- 

 sorbed by the atmosphere, but their photographs show 

 that it is pervious to the chemical rays. Those from 

 Sirius, the nearest and brightest of the stars, travelling 

 through 180 millions of millions of miles and decreasing/ 

 in quantity inversely as the square of the distance,J 

 still have sufficient energy to give a perfect photo- 

 graphic impression of its spectrum ; but Sirius is sixty 

 times larger than the sun, and is many times more 

 luminous. A photograph of the spectrum of Capella 

 has been taken, though three times more distant than 

 Sirius. Photographs of double stars of the sixth and 

 seventh magnitude show that actinic rays from im- 

 measurable distances in space have power sufficient to 

 decompose matter in unstable equilibrium on the sur- 

 face of the earth. 



The chemical power of the moon's light only sur- 

 passes that of Jupiter in the ratio of 6 to 4 or 5, and 

 Jupiter's light has twelve times more actinic energy 

 than that of Saturn. For such comparisons a standard 

 of photographic intensity is requisite. 



A paper coated with chloride of silver can be pre- 

 pared which has a constant degree of sensitiveness, 

 and Dr. Roscoe has proved that a constant dark tint is 



