SECT. XXIV. THERMIC SPECTEUM. 215 



exist in the solar spectrum independent of those of light ; when 

 he used a prism of flint glass, he found that the warm rays are 

 most abundant in the dark space a little beyond the red extremity 

 of the spectrum, that from thence they decrease towards the 

 violet, beyond which they are insensible. It may be concluded 

 therefore, that the calorific rays vary in refrangibility, and that 

 those beyond the extreme red are less refrangible than any rays 

 of light. Since Sir William Herschel's time it has been dis- 

 covered that the calorific spectrum exceeds the luminous one in 

 length in the ratio of 42 to 25, but the most singular phenomenon 

 is its want of continuity. Sir John Herschel blackened the 

 under side of a sheet of very thin white paper by the smoke of a 

 lamp, and, having exposed the white side to the solar spectrum, 

 he drew a brush dipped in spirit of wine over it, by which the 

 paper assumed a black hue when sufficiently saturated. The 

 heat in the spectrum evaporated the spirit first on those parts of 

 the paper where it fell with greatest intensity, thereby restoring 

 their white colour, and he thus discovered that the heat increases 

 uniformly and gradually throughout the luminous spectrum, and 

 that it comes to a maximum and forms a spot at a considerable 

 distance beyond the extreme red. It then decreases, but again 

 increasing it forms a second maximum spot, after which it ceases 

 altogether through a short space, but is again renewed and forms 

 two more insulated spots, and even a fifth may be traced at a 

 little distance from the latter. These circumstances are probably 

 owing to the absorbing action of the atmospheres of the sun and 

 earth. " The effect of the former," says Sir John, "is beyond our 

 control, unless we could carry our experiments to such a point of 

 delicacy as to operate separately on rays emanating from the 

 centre and borders of the sun's disc ; that of the earth's, though 

 it cannot be eliminated any more than in the case of the sun's, 

 may yet be varied to a considerable extent by experiments made 

 at great elevations, under a vertical sun, and compared with 

 others where the sun is more oblique, the situation lower, and 

 the atmospheric pressure of a temporarily high amount. Should 

 it be found that this cause is in reality concerned in the produc- 

 tion of the spots, we should see reason to believe that a large 

 portion of solar heat never reaches the earth's surface, and that 

 what is incident on the summits of lofty mountains differs not 

 only in quantity but also in quality from what the plains receive. 



