CALORIFIC ABSORPTION. 103 



of such light upon it by a convex lens, and it is still unmoved. We are therefore for- 

 ced to conclude that the rays exciting heat are independent of those exciting vision ; 

 that neither the red, nor the yellow, nor the blue possesses inherent caloric ; and, more- 

 over, that substances may be transparent to red, to yellow, or to blue light, or to all, and 

 yet more or less opaque to the rays of heat. 



400. It is not alone among watery solutions or alcoholic tinctures that we find 

 abundant instances of this kind of action; the mineral kingdom furnishes many. A 

 very thin lamina of pitch is transparent to red light, but almost opaque to the rays of 

 heat. I have examined a variety of bodies, gaseous, liquid, and solid, and shall here 

 point out the method which has been followed in obtaining the results contained in the 

 following table. 



401. The mirror being placed upon the shutter, as in (388), a plano-convex lens is 

 to be screwed into the tube, so as to bring the rays to a focus on one of the balls of a 

 very delicate differential thermometer ; the motion of the fluid is rapid, and the instru- 

 ment soon attains a position of equilibrium : this gives the heat of the sunbeam as con- 

 centrated by the lens. To find the effect of any liquid medium in absorbing these rays, 

 the trough filled with the substance under trial is placed at the extremity of the brass 

 tube, in a position as at c (Jig. 50). The cone of rays converges from the lens, a, on 

 the ball b ; but because the trough has plain and parallel surfaces, the rays still pass on, 

 and form an image on the focal ball of the thermometer. The total effect, as given by 

 the expansion of the air in the instrument, and which has formed the basis of the fol- 

 lowing table, is not, however, an exact estimate of the action of the liquid solution. In 

 the instrument which I am in the habit of using, the convex lens is of flint glass, and 

 the plates of the trough of Boston crown glass ; there are, therefore, at least two disturb- 

 ances, the absorbing action of the former, and still more powerful effect of the latter. 

 It has been considered, from the experiments of MELLONI, that the power of absorption 

 was inversely as the power of refraction, but whether an extended train of investiga- 

 tions will corroborate this supposition, remains to be seen. In the following experi- 

 ments, the instrumental arrangement being always identical, a comparison may be insti- 

 tuted of the action of any two of the solutions ; but the absolute action of each cannot 

 be determined, except after allowing for the additional effect of the flint glass lens and 

 the crown glass plates. In practice, it will be found very useful to blacken the focal 

 ball of the thermometer, as seen at b (fig. 50). It serves to give a larger scale of ther- 

 mometric expansion. It is also requisite to cover the thermometer with a very thin case 

 of pure and transparent glass, which prevents the disturbance of currents, and also of 

 the heat radiated from other bodies in the vicinity ; this introduces, however, the ab- 

 sorptive action of a third plate of glass ; b d is the thermometer, and e e the glass cover 

 (fig. 50). 



402. By these arrangements, it was found that a thin stratum of pitch enclosed be- 

 tween two plates of crown glass, and which transmitted a homogeneous red light, ab- 

 sorbing all the other colours of the spectrum, allowed only nineteen rays of heat to pass 

 through of every hundred that fell upon it. 



403. A solution of the sulphate of copper and ammonia, which absorbs the red and 



