the rays, as at S, Leslie determined the reflecting powers of several bodies as 

 follow : 



Brass 100 



Silver 90 



Tin-foil 85 



Block tin , 80 



Steel 70 



Lead 60 



Tin-foil, softened with mercury 10 



Glass 10 



Glass, coated with wax or oil 5 



If these results be compared with, the table of radiating powers in page 476, 

 it will be found that, generally, those substances which are the best radiators 

 are the worst reflectors, and vice versa. In fact, in proportion as the radiating 

 power is increased, the reflecting power is diminished. This analogy is fur- 

 ther confirmed by the fact, that the reflecting power is increased by every in- 

 crease in smoothness or polish of the reflecting surface ; while, on the contra- 

 ry, this cause, as we have seen, diminishes its radiating power. The effect 

 of coating the reflector with a thin film of jelly or other substance has, in con- 

 formity with the same analogy, exactly a contrary effect to that which such a 

 coating produced on radiation. It was found that, as the thickness of the coating 

 increased to a certain limit, the intensity of the radiation was likewise increased. 

 On the other hand, in the case of reflection, the intensity of the reflection is 

 diminished in proportion as the thickness of the coating is increased. 



Let us now consider the effect produced on the focal ball, which will lead 

 us to determine the different powers of absorption which different bodies pos- 

 sess. In all the experiments to which we have hitherto alluded, the focal ball 

 has presented a polished surface of glass, and the effect produced on a ther- 

 mometer, other things being the same, has depended on the absorptive power 

 of the glass over the heat incident upon it. When radiant heat strikes on the 

 surface of different substances, we have seen that a portion of it is reflected, 

 and that this portion varies according to the nature of the substance and ac- 

 cording to the state of the surface. It is clear that all that portion of the inci- 

 dent heat which is not reflected must be absorbed ; and we are led, therefore, 

 by analogy to the inference that, in proportion as the reflecting power of a sur- 

 face is great, its absorptive power is small, and vice versa. 



To bring this inference to the test of experiment, let the ball of a thermome- 

 ter be coated with tin-foil, which was found to be one of the best reflectors. 

 If the side of the vessel coated with lampblack, while the focal ball is covered 

 with tin-foil, be now presented to the reflector, the thermometer will only indi- 

 cate 20, whereas it indicates 100 when the surface of the ball was uncovered. 

 If the bright side of a canister be presented to the reflector when the focal ball 

 is uncovered, the thermometer indicates 12 ; but, if the focal ball be covered 

 with tin-foil, it will indicate only 2^-. Thus we see that the anticipation of 

 theory is confirmed. If the surface of the tin-foil be rubbed with sand-paper, 

 so as to render it rough, and therefore to diminish its reflecting power, its ab- 

 sorbing power will be increased, and the effects on the thermometer will be 

 likewise augmented. Like experiments performed on other bodies lead to the 

 general conclusion, that the absorptive power of bodies increases as the reflect- 

 ing power decreases. 



Since the radiating power of a surface is inversely as its reflecting power, 

 it follows, also, that the power of absorption is always in the same proportion 

 as the power of radiation. In reference to their power of transmitting light, 

 bodies are denominated transparent or opaque. A body which is pervious to 

 ; light is said to be transparent, and one which does not allow light to pass 

 through it is said to be opaque. Transparency is also a quality which bodies 

 possess in different degrees : some, such as glass, water, or air, being almost 



