RADIANT HEAT. 367 



In some of Melloni's experiments, rays from the lamp transmitted 

 in different proportions by various screens, and then equalized, were 

 afterwards found to be transmitted by alum in similar proportions. 

 This he describes by the expression that "they possess the diather- 

 mancy peculiar to the substances through which they had passed." 

 Yet the fact surely implies no new property communicated to the 

 rays. It merely shows, that as different specific rays out of the com- 

 pound beam were transmitted in each case by the first screen, alum, 

 though impervious to the lower heating rays, (i. e., of lower refran- 

 gibility or longer wave-length,) is permeable to those higher rays; 

 and in different degrees according to their nature; an effect simply 

 dependent on the heterogeneity of the compound beam from the 

 flame. Again, with differently colored glasses peculiar differences of 

 diathermancy were exhibited with the rays from a lamp, incandes- 

 cent metal, and the sun; but not more various or anomalous than the 

 absorption of specific rays of light by such media. And besides 

 these considerations, it must be borne in mind that a smooth blackened 

 surface is itself unequally absorptive for the different rays, acting (from 

 its color) more energetically on those of a refrangibility within the 

 limits of the visible spectrum, and which affect the eye as rays of 

 light; and more feebly on the rays of lower refrangibility, and which 

 act more energetically on bodies with reference to the absorptive 

 texture of their surfaces. 



As (according to my experiments) the solar heat is wholly of that 

 kind which is freely transmitted through all colorless media along 

 with the light, it does not appear that there would be any particular 

 advantage in operating on the solar spectrum with a rock-salt prism. 

 Melloni, however, with such a prism found, on interposing a thick 

 screen of water, the most heating rays (i. e., those toward the red 

 end) intercepted, (as they are known to be by water,) and this caused 

 the position of the maximum to be apparently shifted higher up the 

 spectrum, even to the position of the green rays. 



On the other hand, many colored glasses he found absorbed the 

 rays in various proportions, yet they left the point of maximum heat 

 unaltered, i. e., though variously absorptive for the higher rays, they 

 were not of a nature to stop the lower or most heating rays. 



It appears also to be questioned whether the solar beam does- not 

 actually contain some rays of Species I, that is, of wave-lengths 

 greater than any of the prismatic rays, including the ordinarily invisi- 

 ble extreme red. My original experiment on this point, above referred 

 to, may probably be unworthy of comparison in accuracy with those 

 which may now be made with the thermo-multiplier. 



It would therefore be highly desirable if an experiment on the same 

 principle as mine, viz: a black and white thermoscope exposed to- 

 gether, first with and then without a screen, were repeated on the 

 solar rays, with a variety of screens, including especially rock-salt, 

 with all the increased accuracy and sensibility now attainable by the 

 use of two ther mo -multipliers, by which the differences or identities of 

 ratio in the two cases would be rendered evident in the most 



