OF ARTS AND SCIENCES. 163 



thus exceedingly delicate experiments will be necessary to determine 

 the exact relation. Still the intervals are very much larger than the 

 probable error of the determination of a point, so that we may feel 

 assured of the discovery of a relation which further experiments will 

 perhaps determine quantitatively. And we must remember, as in the 

 other experiments, that these measurements are very small and diffi- 

 cult to make ; and the relation here indicated may prove, upon exten- 

 sive examination, to have little or no scientific value. Further and 

 more delicate experiments upon this point are, however, contemplated. 



There is another relation between the curves of Plate IV. that is of 

 considerable interest in its bearing on the theory of exchanges. If 

 we compare first the curve of the black substance CuO and the white 

 substance A1 2 3 , we find the part of the former that corresponds to 

 the luminous spectrum very much fuller than that of the latter ; that 

 is, the substance that absorbs nearly all the light rays at the ordinary 

 temperature emits these rays most copiously when heated. 



The green oxide of chromium, which is intermediate in color, and 

 absorbs all but the green rays at the ordinary temperature, is also 

 intermediate in its curve and emits least of all the green rays when 

 heated. So the red oxide of iron is intermediate between the black 

 and the green. But the curves also show us that we are not war- 

 ranted in extending conclusions, drawn from observations of the 

 luminous spectrum, to other parts. 



In concluding this paper there is a strong temptation to speculate 

 upon the meaning of the results obtained. That the geometrical form 

 of the curve should be so nearly the same at all temperatures, and of 

 the same general form for all substances, is a fact that probably must 

 have an important physical interpretation. Does not the similarity of 

 the curves for different substances show a similarity of movement of 

 the ultimate components of the several substances, and so point to a 

 similarity of ultimate composition of all matter, the slight differences 

 in the grouping of these parts giving rise to the comparatively slight 

 variations from the same form ? Certainly this is not proof, but is it 

 not evidence ? And is it not probable that the superposition upon the 

 radiations from the ultimate atoms, of the radiations from the group- 

 ings of these atoms, should cause the curve, as a whole, to move 

 slightly to a shorter or longer wave-length, as the weight of a group 

 is lighter or heavier ? But I am aware that such speculations are 

 founded on too insufficient data, and I offer these results merely as 

 an experimental contribution to the science of radiant energy. 



