Action of the Atmosphere ujion neichj-deepcncd Soil. 421 



light, and consequently on the tops of hiijh hills the celestial 

 luminaries are seen to shine with a lustre unknown to the inhabit- 

 ants of tlie plains ; and I recollect receivinir a description of the 

 wonderful brightness presented by the morning star from one who 

 had viewed it from the summit of one of the loftiest peaks in 

 Spain. This diminution of the light is referred by Clausius to 

 multitudes of fine vesicles of water, which even in serene weather 

 float in the air and produce reflection. 



Seeing that, with the exception of some observations and expe- 

 riments by Petit and Scheele, little had jjeen done towards the 

 investigation of the chemical action of the solar radiations until 

 the present century, we cannot but suppose that we are only yet 

 beginning to be acquainted with their peculiar influence, and our 

 present knowledge of the changes induced by them is confessedly 

 very imperfect. 



The attention however of some of the ablest philosophers and 

 chemists of the day is at present keenly directed to the subject, 

 and additions to our knowledge are continually dropping in. 

 What has been done does not bear very decidedly upon -the sub- 

 ject of the present paper, but is still worthy of attention. Ritter 

 of Jena, in 1801, demonstrated the existence of rays beyond the 

 spectrum which have no luminous power, but exhibit very active 

 chemical agencies, and he inferred the existence of two sets of 

 invisible rays, the least refrangible favouring oxygenation, whilst 

 the most refrangible deoxidise, and later observers have con- 

 firmed his results. It was generally supposed that the chemical 

 agency was confined more particularly to the blue and violet 

 rays. To show the disproportion which exists in this respect 

 between the energies of difierent rays, Berard " concentrated by 

 means of a lens all that part of the spectrum which extends from 

 the green to the extreme violet, and he concentrated by means of 

 another lens all that portion which extends from the green to the 

 extremity of the red. This last pencil formed a white point so 

 brilliant that the eyes were scarcely aljle to endure it ; yet the 

 muriate of silver (a salt highlv susceptible to the action of light) 

 remained exposed more than two hours to this brilliant point of 

 light without undergoing any sensible alteraticm. On the con- 

 trary, when exposed to the other pencil, which was much less 

 bright and less hot, it was blackened in less than six minutes." 

 (Ami. dc Chim., vol. Ixxxv. p. '60d.) 



Chemical action has however been traced to every part of the 

 prismatic spectrum, althou^-h the least luminous seem mos 

 powerfully to affect inorganic bodies. 



The heating agency may, in like manner, be separated from the 

 luminous. Mclloni, writing in 1835, says the rays can be passed 

 through certain media " which absorb the whole of the calorific, 



