51G STATE BOARD OF AGRICULTURE. 



resiJectively. The diflFerence between the bhiek and white on July 27 

 was G.o° C. and on Anj^ust 5.0° (\ The differences l)ctween the other 

 colors are not so great, esi)ecially between the black and the blue, the 

 yellow and the white, the red and the green, 



Tlie minimum figures, on the other hand, show very conclusively at 

 both times that the 1em]ierature of all the different colored sands is 

 practically the same, the difference in any extreme not buing more than 

 .3° C. and in favor of the sands which had attained the highest tem- 

 perature. These results are interesting because they lend further con- 

 firmation to the conclusions alroady announced that color has no effect 

 upon radiation. This conclusion, liowever, may be objected to because 

 if the different colored sands attained different temperatures during the 

 sun insolation because of their different absorptive powers, while dur- 

 ing the night they all cooled to the same point, then they radiated 

 differently, and therefoi'e those colors which caused the highest absorp- 

 tion lost the most heat. This is all true but those sands which lost the 

 most heat did not do so on account of the greater radiation of their 

 color, but rather because of the difference between their temperature 

 and that of the air. The law of cooling as announced by Newton states 

 that the rate of cooling of a body is proportional to the ditference in 

 temperature between its own and that of the surrounding medium, 

 while the similar law of Stefan states that the rate of cooling or radia- 

 tion of a body is proportional to the fourth power of its absolute tem- 

 perature. 



Theoretically speaking color should have no effect upon radiation be- 

 cause all colors emit in dark, rays of low refrangibility which are the 

 same while the rays they absorb from the sun are of high refrangibility 

 and are different. The different radiating power observed in tlie differ- 

 ent colored substances is probably due, therefore, to the difference in 

 their composition rather than to their color. 



RADIATING POWER OF NATURAL SOILS. 



The radiating power of different soils was studied under three differ- 

 ent conditions or treatments: (1) in dry and powder state; (2) with 

 moist subsurface and dry surface; and (3) in the natural condition. 

 For convenience these headings will be considered in reverse order. 



As has already been stated the radiating power of soils has been 

 studied thus far only by Ahr.^* He conducted experiments on the 

 radiating capacity of different kinds of soils in the dry. moist, and wet 

 condition. In the dry condition he obtained the following results: 

 quartz 96.5, loam 91.4, calcareous sand 94.1, kaolin 91.5, and humus 

 89.8%. In the moist and wet condition the values were higher but were 

 the same for all the soils. 



In the investigation of heat conductivity, natural cores of soils were 

 emi»loyed. These same cores of the different types of soil were also 

 used in the study of radiation with the object of arriving at moi-e defi- 

 nite and practical conclusions as to their radiating power. 



The study was conducted by taking oft' both covers of the box con 

 taiuing the core, pushing the latter very gently with a block of board 



I (14) Forsch. a. d. G. d. Agrik. Phy. XVII, 1894. 



