EXPEE^IMENT STATION BULLETINS. 



519 



lion; (2) to see Iiow this compared with their radiating power in their 

 moist state; and (.'{) to compai-e tlieir radiating power with their lieat 

 transmitting power, in their air dry condition. Tlie iiivestigati(m was 

 conducted in the same manner as the preceding one witli Ihe exception 

 that in the present case all the soils were dry and their particles wei-e 

 of about the same size, 1. e., they passed a 40 mesh sieve. This latter 

 precaution was taken because preliminary experiments showed that 

 radiation tends to increase slightly with the increase in size of parti- 

 cles. The results obtained are given in the following table: 



This table shows that the radiating power of the different soils in 

 the dry condition is different, that the sand shows the highest radia- 

 tion, followed in order by the gravel, clay, loam and peat, respectively 

 and that the difference between the first and last soil is very marked. 

 This latter fact is highly important because it lends further evidence tft 

 the theory that radiation is independent of color. The table also shows 

 that water exhibits the greatest radiating power. This would point 

 to the conclusion that it is the water which increases so markedly the 

 radiating power of moist soils, and which overshadows their radiation 

 possessed in their dry state. This greater radiating capacity of water 

 is contrary to the common belief. The widely held opinion is that water 

 radiates its heat with greater difficulty than any soil material. The 

 truth of the matter is, however, that water loses its heat slowly, not 

 because of its low radiating power but because of its high heat capacity. 



It has been stated that the radiating power of these different soils 

 in their dry condition was investigated partly in order to compare it 

 with their heat conducting power. The results show that the two prop- 

 erties run together, a fact contrary to the statements of some physicists. 



It now remains to consider the practical signiflcance of the forego- 

 ing results. The studies on the specific heat (of dry soils) and heat 

 transference in natural soils led to the conclusion that these two prop- 

 erties play a small part in determining the temperature of the differ- 

 ent soils, especially during the important seasons of the year. Radia- 

 tion plays still a smaller part. This inference is inevitable in the light 

 of what has already been obtained. It has been conclusively shown 

 that all the different types of soil when well moistened possess exactly 

 the same radfating power, and when they have a thin layer of dry mulch 

 this power varies only slightly. This means then that all these soils, 

 under field conditions, cool at the same rate as far as their property of 

 radiation is concerned, the different rate of cooling and warming that is 

 actually observed among these soils is, therefore, due mostly to their 

 different moisture contents and hence to their different specific heats. 



