THIRD SERIES.— REFRANGIBILITY OF HEAT. 205 



80, We can, therefore, in this way form but an imperfect idea of the compa- 

 rative homogeneity of the different kinds of heat. Such comparisons can only be 

 made advantageously by comparing the results obtained in immediate succession 

 from one and the same somxe with interposed screens of different qualities, as in 

 the comparison which we instituted between heat direct from Locatelli's lamp, and 

 that transmitted by glass, (art. 63). 



81. The facts respecting refrangibility, which may now be considered as ascer- 

 tained, serve to render our ideas much more precise in several respects. For in- 

 stance, (1.) the range of mean refractive indices for heat is small, all the modifica- 

 tions which we have considered lying within a range of .04, or between 1.51 and 

 1.55 nearly, which is little more than the commonly assigned dispersion of light, 

 which, for rock-salt, is between the limits 1.54 and 1.57 nearly. This, however, 

 is for extreme rays of light, which can hardly be said of heat ; the extremes of dis- 

 persion are certainly much wider apart. (2.) The mean refractive index of direct 

 rays from different sources varies surprisingly little. In fact the differences for 

 direct rays of heat from the Locatelli-lamp, incandescent platinum, and from a 

 crucible heated to 450°, seem almost insensible, or within the limits of error of 

 experiment. It is to be recollected, however, that this is compatible with the ut- 

 most variety in the composition of each. (3.) The effect of interposed screens in mo- 

 difying the transmitted heat is very remarkable. These, so far as I have tried them, 

 invariably raise the index of refraction, (alum, glass, opaque glass, and opaque mica 

 for the Locatelli-lamp ; glass and opaque mica for incandescent platinum, and clear 

 mica for dark heat). This is the case even with those substances which suppress 

 light altogether, and which therefore cannot be considered to do more than de- 

 tach the heat of considerable refrangibility from the light which usually accom- 

 panies it, not as stopping the most refrangible rays and admitting the passage of 

 those of lower temperature. Probably no substance acts in this way, though some 

 (as black glass and mica, as the experiments of Melloni indicate) may probably 

 absorb the heat spectrum at both extremities. It is probably to this source that 

 we must attribute the very small fraction of heat transmitted by the black glass 

 I used, being only that constituting the rays of the higher degrees of refrangibility, 

 all those of low and mean, and also of the highest, degrees of refrangibility being 

 probably absorbed. (4.) With respect to the homogeneity of different kinds of heat, 

 I have already stated that we can deduce nothing certain from the forms of the 

 curves in Plate XII. They confirm, however, a view which I have long entertained, 

 that heat from non-luminous sources is more homogeneous than any other. I 

 argued this partly on the ground stated in art. 40 of this paper, and still more 

 from the uniformity of results which I have in all classes of experiments obtained 

 from dark heat, which often more than made up for the narrower range of the 

 thermal effect, and which shewed that the discrepancies observed in other cases 

 were due not so much to errors of observation, as to unavoidable changes in the 



