186 Mr. Poicell on Terrestrial Light and Heat. [Sept. 



why is any part of the total effect intercepted ? And that this 

 is the case, may be readily seen in the latter cases of De la 

 Roche's experiments, as well as in many others. 



If it be merely light into which, on Prof. Leslie's hypothesis, 

 the radiant heat of the body has been gradually converted, the 

 whole effect ought to be transmitted ; since the diminution 

 which light suffers in passing through a plate of clear glass 

 would be very trifling. 



(11.) All these and any similar questions which might arise, 

 appear to me capable of solution by merely extending the 

 inquiry as to the connexion subsisting between the interceptive 

 power of glass in different instances, and the relation which 

 both the intercepted and the transmitted portions of the effect 

 bear to the nature of the surfaces on which they act. By obvi- 

 ous experimental methods, we might examine whether the trans- 

 mitted part affects differently coloured, and differently textured, 

 surfaces in the same proportions as the total or direct effect 

 does. And each may be compared with what we know of the 

 properties in reference to these points, by which both simple 

 radiant heat, and the heating power of the solar light, are cha- 

 racterized. Thus we may clearly ascertain by the application 

 of definitive tests, whether the portion intercepted be similar to 

 simple heat in its relation to the texture of the surfaces on 

 which it acts most powerfully ; if so, it agrees with the " ra- 

 diant heat" from non-luminous sources in the two charac- 

 teristics by which it may be defined. If again, the transmitted 

 portion agree with the solar light in acting more energetically 

 on surfaces of a darker colour, this would prove its joint pos- 

 session of these two properties whose combination marks the 

 " heating power of light." Such an examination would thus 

 decide whether the radiating influence be due to one simple 

 agent or to the joint, though distinct, operation of two. 



(12.) In describing my observations on the solar rays, I 

 alluded to the extension of the same examination to other cases. 

 This constitutes one principal part of the experiments referred to 

 in the present instance. 



Two thermometers coated, one with chalk and the other with 

 Indian ink, were exposed precisely in the same way as before, 

 both with and without a screen of plate glass to the radiation 

 from non-luminous hot iron ; from the same iron at a bright red 

 heat, and from the flame of an Argand lamp. Instead of the 

 same ratio in the effects on the white and on the black bulb 

 being preserved, as in the case of solar heat, I found a very 

 considerable difference. When the rise of the two thermome- 

 ters was observed xoith the screen, the ratio subsisting between 

 them was very considerably greater than that which was ob- 

 tained without the screen. The same thing was also tried in 

 other ways by several applications of the differential thermo- 



