408 Action of the Atmosphere upon newly-deepened Soil. 



4. The pliosphorogenic, or power of inducing phosphor- 

 escence. 



Other properties, for aught we know, may he induced, such as 

 magnetism, and perhaps more not yet investigated. 



" If we decompose," says Melloni, " a bundle of solar rays by 

 means of a rock-salt prism, and measure the degree of heat 

 proper to each band of the spectrum, we find that the tempera- 

 ture increases from the violet towards the red until it has reached 

 a point midway between the red and the yellow ; at this point a 

 pretty rapid decrease of temperature takes place." Light and 

 heat, he concludes, proceed from two distinct causes which, 

 perhaps, are but different effects of a single cause — the luminous 

 and the calorific rays being perhaps only two essentially dis- 

 tinct modifications which the etherial fluid suffers in its mode of 

 existence. 



Of these properties, the power of imparting heat is the one 

 that most powerfully affects the materials of the soil, and as the 

 effects produced are mainly due to the alternations of heat and 

 cold brought about by the vicissitudes of day and night, summer 

 and winter, it will be necessary to investigate what are the con- 

 ditions of temperature to which the soil is exposed both at the 

 surface and at moderate depths. 



Professor J. D. Forbes has shown (Proceedings of Royal Society, 

 iv. 391) that the solar rays are subject to considerable diminu- 

 tion in their passage through the atmosphere. The law of the 

 absorption of the incident heat rays probably depends on a 

 difference in their nature, their elements being partly absorbable 

 and partly persistent, the absorption in passing through a ver- 

 tical atmosphere of 760 millimetres of mercury (=304 Eng. 

 inches) being such as to reduce the incident heat from 1 to 0'534. 



The amount of absorption of incident solar heat traversing the 

 atmosphere vertically in clear weather is, according to — 



Bouguer .. .. 19* per cent, of that incident on the exterior 



of the atmosphere. 



Lambert .. .. 41* ditto ditto 



LesUe 25" ditto ditto 



Kiimtz .. .. 32* ditto ditto 



Pouillet .. .. 25- ditto ditto 



Kiimtz and Forbes 29* ditto ditto 



Taking the loss of radiant heat in its vertical passage through 

 the air at only 25 per cent., at an angle of elevation of 25° tiie 

 force of the sun's heat would be reduced to one-half, and at 5° to 

 one twentieth part. The difference of the direct effect of a vertical 

 and horizontal sun is due to this cause alone, exaggerated im- 

 mensely by the variable meteorological state of the atmosphere. 

 {Forbes, Brit. Assoc. Beportfor 1840.) 



