Improvements in Science. 129 



other ; and, in general, we may say, that the magnetic 

 power, produced by a discharge of the Leyden jar, is only 

 the effect determined by the simultaneous union of two 

 magnetizing, more or less unequal and opposed, forces. 

 7. The common simple spark of the machine produces 

 analogous phenomena. 



IV. HEAT. 



Reflexion of radiating heat. — The researches of Leslie and 

 Rumford have shewn, that rays of heat are reflected by 

 bodies, according as the surfaces are more or less polished. 

 But a natural question now presents itself, viz. What, 

 in each case, is the proportion between the quantity of re- 

 flected and incident heat ? The results obtained by Melloni 

 on the immediate transmission of radiating heat, through 

 many solid and liquid substances, afford a resolution of this 

 question.* When calorific rays fall perpendicularly on 

 the anterior surface of a diathermanous plate, possessing 

 parallel faces, they undergo a certain reflexion, penetrate 

 into the anterior, are partly absorbed, arrive at the second 

 surface, are there again reflected, and pass out again into 

 the air, pursuing their first direction. But in certain cases, 

 there is no continual absorption, and where, consequently, 

 the difference between the quantity of incident heat and 

 the quantity transmitted is exactly equal to the value of 

 the reflexions produced upon the two surfaces of the plate. 

 Rock salt affords an excellent example of this. Plates of 

 this substance, in a pure state, and well polished, transmit 

 0-923 of the incident heat, whatever be their thickness, and 

 the nature of the rays of heat, or the modifications which 

 these ravs may have undergone, in their passage through 

 other plates. Let us suppose two plates of rock salt, the 

 one "0154 inch in thickness, the other -154. Then, from 

 what has been said, it is obvious, that the transmission of 

 the first plate will be equal to that of the second; and, if 

 we suppose the first of these plates divided into lOlayers, each 

 •0154 inch in thickness, the absorbing power of the nine 

 posterior layers (each "0154 inch) will have no appreciable 

 effect. Hence, if the rays undergo any absorption, it must 

 take place in their passage through the first layer. Let us 



• Institiit. No. ISO. 

 VOL. III. K 



