RADIANT HEAT. 305 



% Ratio of effects on 



Towards hot body. thermometer. 



Glass 5 



Amalgam 35 



Amalgam, blackened 92 



Amalgam removed, — glass blackened 180 



(Fssai, &c, p. 72.) 



2.) He tried to refract simple heat without effect. 



Sir W. Herschel tried with a lens, and supposed it effected: this 

 has been refuted by Sir D. Brewster. — {Vide infra; Phil. Trans. 1800, 

 Part II, No. 15, Exp. 19, 20.) 



3.) Professor Leslie's experiments on screens are perhaps the most 

 valuable portion of his inquiry. 



He found the effect of a screen increase rapidly with its distance 

 from the source, (p. 28,) and less so with its thickness, (p. 38.) 



Different substances appear to have a different interceptive power: 

 but this upon examination appears always to be dependent on their 

 conducting power, and the absorptive nature of their surface jointly. 



The most decisive experiment on this point was that made with 

 two panes of glass, each having one side coated with tin foil; accord- 

 ing as the plain or coated sides were placed in the contact, the com- 

 pound screen had a greater or less apparent interceptive power; that 

 is, a greater or a less power of absorbing and subsequently radiating 

 the heat. Again, either might be used separately, or the two at an 

 interval. — (p. 35.) 



4.) Prevost concluded that a certain portion of heat is directly trans- 

 mitted through transparent screens, by employing moveable screens 

 which continually presented a fresh surface, so that it was supposed 

 all communication of heat and conveyance by way of secondary radi- 

 ation would be prevented. 



But it must be considered that it is impossible to prevent entirely 

 any portion of a screen in the most rapid motion from acquiring heat:, 

 no such experiments therefore can be strictly conclusive. 



Dr. Ritchie tried experiments with the same view, by means of a 

 film of liquid adhering to threads stretched across a frame continually 

 renewed. — (Phil. Trans. 1827, Part II, p. 141.) But to this a, similar 

 objection must apply. 



5.) The results of Professor Leslie do not apply to temperatures 

 above those of boiling water. 



This extension of the inquiry formed the subject »f the researches 

 of De la Roche. The complete account of these is given in its 

 proper place; at present we have to consider them only as far as 

 relates to bodies below luminosity. He tried the effect of a screen 

 of glass, first transparent, and then with one surface blackened, on 

 the heat radiating from mercury at 180° centig. and at 346 when it 

 was boiling. — (Biot, Traite de Phys., iv, 640.) 



The results were as follows: 



20 



