EXPERIMENT STATION BULLETINS. 511 



EFFECT OF COLOR ON KADIATION AND THE RADIATING 

 POWER OF SOILS UNDER DIFFERENT CONDI- 

 TIONS OR TREATMENTS. 



OBJECT AND METHOD OF EXPERIMENTATION. 



Radiation has already been defined as a process in which heat is 

 propagated as a free-wave motion without permanently affecting the 

 intervening space between the radiating body and the body receiving 

 (he radiant energy. The phenomenon is likened to light. Like the latter 

 it is propagated in straight lines; it is reflected and refracted; it travels 

 with great rapidity; and obeys the same laws of propagation. 



Our knowledge concerning the rate of radiation or of cooling of sub- 

 stances is unsatisfactory. Newton seems to be the first who performed 

 experiments on the subject. From the data he obtained he enunciated 

 the law to the effect that the quantity of heat lost or gained by a body 

 in a second is proportional to the difference between its temperature 

 and that of the surrounding medium. Dulong and Petit, however, 

 proved that this law is not general but applies only with differences 

 of temperature which do not exceed 15° to 20°. Beyond this, the quant- 

 ity of heat lost or gained is greater than what is required by this law. 



In 1879 Stefan showed, from his own researches and from recalculating 

 his predecessors' data upon the subject, that the rate of radiation of a 

 body is proportional to the fourth power of its absolute temperature. 



On account of the great complexity of the phenomenon and of the 

 insurmountable difficulties involved in the technic, the radiating power 

 of only a few substances has been determined. This limited study, 

 however, has conclusively revealed that the different materials have 

 different radiating or emissive capacity. This is also true of the same 

 substance in different conditions and in different temperatures. 



In the investigation here presented the main object has been to dis- 

 cover the radiating power of different soils under various conditions 

 or treatments and thereby see, if possible, to what extent radia- 

 tion affects the temperature of soils. As will be stated later, this sub- 

 ject has remained practically unexplored. Indeed, there has been only 

 one man who has attempted to investigate it. 



The different soils experimented with were quartz sand, gravel, loam, 

 clay and peat, — the true representatives of the most common soils. 



These soils possess, among other characteristics, distinctive differ- 

 ences in color. It is a common opinion that color has a very marked 

 effect upon the radiation as well as upon the absorption of heat. It 

 is claimed that radiation and absorption are closely related, that the 

 highest absorbers are also the highest radiators and the lowest ab- 

 sorbers are the lowest radiators. A black substance for instance being 

 able to absorb practically all the heat rays on account of its low re- 

 flection, low diffusive and high absorptive power, is a high heat ab- 

 sorber and consequently a high heat radiator. The opposite is true 

 of a white substance. A white substance on account of its high re- 

 flective, high diffusive and low absorbing power, is a low absorber and, 

 therefore, a, low radiator. 



