398 Intelligence and Miscellaneous Articles. 



sum of the heats observed by the thermo-rheometer is proportional to 

 the square of the electricity put in circulation. 



In short, the thermo-rheometer is at once and of itself a rheostat, 

 a galvanometer, and a measurer of electromotive forces ; it is the 

 only one of these various instruments which can be applied to induc- 

 tion-currents as easily as to ordinary currents. — Comptes Rendus, 

 July 6, 1868. 



INVESTIGATIONS ON OBSCURE CALORIFIC SPECTRA. 

 BY M. DESAINS. 



The whole of the researches relative to the calorific spectrum have 

 long ago established that the heat of the luminous part of this spec- 

 trum is more transmissible through water than are the mean obscure 

 rays. Melloni has gone further, and, in a memoir presented in 1831 

 to the Academy, announced that the least-refrangible parts of the 

 obscure solar spectrum are completely destitute of the property of 

 traversing a layer of water a few millimetres thick. These portions, 

 from this point of view, would resemble the radiations from blackened 

 bodies heated to temperatures below 300°. 



Whatever since Melloni's time be the results which science has 

 acquired relative to the properties of solar calorific radiations, I have 

 thought it right to investigate again whether, in the spectra formed 

 of these radiations, there really are rays analogous to those of sources 

 at a very low temperature. 



The ideas recently acquired on the absorbing action of vapours 

 made it little probable ; and in fact I could never find, in the solar 

 spectra I have investigated, rays completely deprived of transmissi- 

 bility through water. These spectra were so pure that the eye could 

 readily detect the principal lines ; the luminous part was about 25 

 millims. in extent, and their obscure part about the same. I exa- 

 mined the different parts by the aid of a line pile, the breadth of which 

 was scarcely a twenty-fifth of the whole breadth of the spectrum. In 

 the maximum I had frequently a deviation of 30°, and sometimes 

 much more ; but on removing the pile to the limits of the obscure, 

 where there was only a deviation of one or two divisions, I found 

 that the heat capable of producing these deviations was transmitted 

 in considerable proportion through a layer of water of 2 millims. 

 It is not, perhaps, superfluous to remark that these latter pencils of 

 obscure heat did not in my experiments exceed the two-hundredth 

 part of the total heat spread over the whole extent of the spectrum. 



Entirely different results are obtained on analyzing the spectra of 

 terrestrial luminous sources, such as the flame of a lamp without a 

 glass, or an incandescent platinum wire. 



In one series of experiments a wire of this kind was kept at a 

 cherry-red heat in a small gas-flame. Its radiation, confined by a 

 suitable diaphragm, fell on a rock-salt lens with a focus of 15 cen- 

 tims., which formed a well-defined image at a distance a little greater 

 than 30 centims. With a fine prism of rock-salt, the rays were 

 deviated and dispersed. The luminous portion of the spectrum was 

 barely visible, and the calorific effects it produced of feeble inten- 

 sity. The dark spectrum was at least as expanded as in the case 



