515 



any plane radiating surface, while the area of that surface can be easily 

 determined to any required degree of accuracy. The quantity of 

 heat radiating under given conditions, from a unit of surface in a unit 

 of time, can thus be easily ascertained. The paper contains a detailed 

 description of the instrument, and of the experiments made with it. 



The following are experimental results thus obtained, the unit 

 of heat being that quantity of heat which would raise 1000 grs. of 

 distilled water 1 Centigrade. The formula is that of Dulong and 

 Petit, where 



0= temperature of the surrounding medium (the air in these ex- 

 periments), expressed in Centigrade degrees ; 



t= the excess of the temperature of the radiating surface above 

 that of the surrounding medium, in Centigrade degrees ; 



p= pressure of the surrounding medium (the atmosphere in these 

 experiments), expressed by the height of the barometer in metres ; 



== 1'0077, a numerical quantity which is always the same for all 

 radiating surfaces and surrounding media. 



Then if Q denote the quantity of heat, expressed numerically, 

 which radiates from a unit of surface (a square foot) in a unit of 

 time (one minute), we have the following results for the substances 

 specified : 



Glass. 



/ p Y 45 



233 



Dry Chalk. 

 Q= 8-613 fl(*- 1 ) + -03720 (-- 2 



Dry New Red Sandstone. 

 Q= 8-377 fl*(-l) + -0372o(lL\ 



Sandstone (building stone). 

 Q= 8-882 e (a' 1) + -03720(JP 

 Polished Limestone. 

 Q= 9-106 V-1) + "03720 (JL 

 Unpolished Limestone (same block as the last). 



