512 STATE BOARD OF AGRICULTURE. 



It is concluded by some soil physicists, therefore, that a black soil 

 attains a very high tem])erature during the sun insolation but in the 

 early morning its warmth is about the same if not lower than that of 

 a light colored soil, 



A number of physicists have investigated the effect of color on radia- 

 tion but. as far as the writer is aware, only two have studied it in its 

 application to soil temperature. Thp first worker Avas Lang.^- His 

 method of study consisted of determining the radiation of a white sub- 

 stance first and then mixing this with a colored substance, such as 

 soot, and determining again the radiation of the mixture. He conclud- 

 ed from his results that color affects radiation and absorption equally 

 well. 



Several years later "Ahr reinvestigated the subject. He employed a 

 slightly different arrangement in the apparatus but used practically 

 the identical material of Lang. He obtained results which were prac- 

 tically the same as his predecessors' but he did not deduct the same 

 conclusions. He thought that the difference in radiation observed in 

 the various colored substances might be due to the composition of the 

 material rather than to the color. 



Since this question was still unanswered, and since it was desirable 

 to know definitely whether color does affect radiation in order to 

 arrive at proper conclusions on the radiating power of the different 

 types of soil already mentioned, it was deemed necessary and advisable 

 to investigate the subject. Such in\Testigation, it appeared, should be. 

 conducted differently from that of the previous investigators if more 

 definite and conclusive results were to be obtained. One of the greatest 

 difficulties that this study presents, is the fact that there is no single 

 substance of the same composition which contains all the different rays 

 of the spectrum. It seemed, however, that this difficulty might be over- 

 come, to some extent at least, by the following procedure: Instead of 

 using different solid materials of entirely unlike composition to obtain 

 the different colors, it was thought best to use the same kind of solid 

 material colored with the different aniline dyes. The solid substance 

 employed was quartz sand and its radiation as affected by color was 

 measured by an apparatus to be described. 



The instruments commonly employed in studying or measuring radia- 

 tion are the thermopile and bolometer. The bolometer is the more 

 sensitive instrument of the two. On account of various reasons, how- 

 ever, it could not be obtained; consequently at the beginning of the 

 work a high grade thermopile with a very sensitive galvanometer were 

 employed. 



The results obtained by this method were unsatisfactory because of 

 the fact that one end of the thermopile was exposed to the room tempera- 

 ture, a slight change in which would change the readings. In order 

 to overcome this difficulty a thermopile was constructed one end of 

 which could be kejtt in ice water and thus kept under a better controlled 

 temperature. This change or improvement, however, proved of no 

 avail as the electomotive force -was so great on account of the large 

 difference in temperature between the two ends that the deflection on 

 the unshunted galvanometer would invariably go beyond the scale. 



(12) Forsch. a. d. G. d. Agrik. Phy. 1. 1S78. 



(13) Forech. a. d. G. d. Agrik. Phy. XVII, 1894. 



