Improvements in Science. 131 



posterior surface of the rock salt ; but if we wish to have 

 the absolute value of this last reflexion, we shall obtain 

 it by substituting -0393 in place of R in the expression 

 R (1 — R), or more simply, by taking the difference be- 

 tween the numbers "077 and -0393, which gives, in both 

 cases, -0377. Experiment afforded similar results with 

 glass, rock crystal, alum, fluor spar, topaz, sulphate of 

 barytes, &c. Melloni concludes, that we may say, as a 

 general expression, that radiant heat experiences a reflexion 

 of about x^ths of the incident quantity, when it falls per- 

 pendicularly upon the surface of diathermanous substances. 

 With regard to the heat reflected by athermanous bodies, it 

 is necessary, first, to observe the effect of the transmission 

 of heat through rock salt, when the radiation, derived 

 from a constant source, is perpendicular to its faces. The 

 plate is then inclined to the incident rays. No sensible 

 diminution in the quantity of heat transmitted is exhibited 

 as long as the inclination does not exceed 30° or 35°. The 

 reflexion of the perpendicular rays is then sensibly equal 

 to that which the rays forming an angle of 55° to 60°, with 

 the reflector, undergo. If now, we throw upon the well 

 polished surface of a very large plate of glass, or rock cry- 

 stal, a compact quantity of radiant heat, at an incident angle 

 of 55° or 60°, and receive the portion reflected in the in- 

 terior of the tube, which surrounds the pile of the thermo- 

 multiplier ; having noted the force indicated by the gal- 

 vanometer, and repeated the same experiment upon the 

 polished surface of an athermanous body, we shall have a 

 second force different from the first. The reflexion of the 

 athermanous body will obviously be equal to the number 

 •0393 multiplied by the proportion of the two forces ob- 

 served. 



The following exhibits a comparison between rock cry- 

 stal and yellow copper. 



Reflexion of rock crystal, .... 3-15 

 Reflexion of yellow copper, . . . 35*63 

 Proportion of the two reflexions, . . 11*30 

 Product of the two numbers, ? -ri44 



0393 and 11-3 S ' ' 



K -2 



