318 Intelligence and Miscellaneous Articles. 



back to a volcanic stage conditioned by its cooling ; and the absence 

 of an atmosphere can be explained by the nature of the materials 

 which, judging from the small specific gravity of that heavenly 

 body, compose its crust and are capable of confining the volcanic va- 

 pours.— Sitzungsh. derWiener AJcademie, math.-naturw . Classe, 1877, 

 No. vii. pp. 62-64. 



RESEARCHES ON HEAT-SPECTRA. BY P. DESAINS. 



In a research published in May 1870 I established that, in the 

 solar spectrum formed by a rock-salt apparatus, the heat accom- 

 panying the luminous rays is about one third of the total heat ; on 

 the contrary, in the spectrum of incandescent platinum it is only an 

 insignificant fraction of it. The results are similar when refracting 

 prisms entirely of flint glass are employed. I tried in vain to cause 

 this difference to vanish by transmitting the radiations from the in- 

 candescent metal through more or less thick layers of water. In my 

 experiments the dark portion of the spectrum of platinum had an 

 extent of about 4° ; the interposition of a layer of water of 1 centim. 

 thickness reduced to 2° the length of this obscure spectrum, and 

 diminished its intensity to nearly three fourths of its primitive 

 value ; but the luminous heat still remained a very small fraction 

 of the total heat. Greater thicknesses of water shortened the dark 

 region so as to leave it only a much less extent than in the solar 

 spectrum. 



Spectra obtained with the electric lamp, on the contrary, may be 

 rendered in their totality much more like those obtained with the 

 rays of the sun. 



In the electric spectrum, at first, heat is found as far as into the 

 blue. De la Provostaye and I verified this fact more than twenty- 

 five years since ; and in some recent experiments, the heat in the 

 luminous part of a like spectrum appeared to me to be about one 

 sixth part of the total heat. It is true this ratio is only half of that 

 found in operating with the solar rays ; but if we pass the radiation 

 of the electric lamp through from 3 to 4 centims. of water, we 

 notably reduce the calorific intensity of the dark portion of the 

 spectrum, almost without modifying the luminous heat ; and this 

 latter then becomes about one third of the total, just as it does in 

 the solar spectrum. 



The spectrum thus obtained is nevertheless not absolutely iden- 

 tical with the solar spectrum ; in particular, it has less extent than 

 the latter, especially at the violet end ; but the curves representing 

 the intensities in the two spectra exhibit only very slight differences 

 from one another throughout the region comprised between the 

 middle of the green and the portion of the dark spectrum sym- 

 metrical with the blue ; and this is especially the calorifically 

 effective region. I will add, in conclusion, that the pile employed 

 in the experiments here described consisted usually of 50 large 

 Bunsen elements; sometimes, however, the number of these 

 elements reached 100. Finally, it will not be useless to call to 

 mind that, according to the usual estimates, the vapour contained 

 in an atmospheric column extending vertically to the limit of the 



