154 Intelligence and Miscellaneous Articles. 



more powerful, but the propagation of the rotation of the molecules 

 feebler, than with an armature of soft iron and a magnet. 



In regard to the numerous details and further results of this 

 painstaking investigation we must refer our readers to the original 

 work. — Beibl'dtter zu den Annalen der PhysiJc und Chemie, 1880, 

 No. 1, pp. 73, 74. 



ON THE THERMAL ABSORBENT AND EMISSIVE POWERS OF FLAMES, 

 AND ON THE TEMPERATURE OF THE VOLTAIC ARC. EXTRACT 

 OF A LETTER FROM FR. ROSETTI TO A. CORNU. 

 Conclusions. — 1. The transparency of flames is very great ; conse- 

 quently flames absorb very little of the thermal radiation which 

 traverses them. If the radiation proceeds from a flame of the same 

 nature, and if the flame through which it passes has a thickness of 

 1 centimetre, the coefficients of transparency and absorption are 

 represented by the numbers 0*865 and 0-135 respectively, for the 

 white flames produced by illuminating-gas, as well as for the pale 

 blue flames of Bunsen burners. 



2. The transparency diminishes and the absorption increases 

 proportionally, if the thickness of the flames is augmented. If the 

 flame has an infinite thickness, it is athermanous ; that is to say, it 

 absorbs all the heat-rays proceeding from a flame of the same 

 nature: the transparency is nil, the absorption equal to unity. 

 But these limits are nearly reached with finite and not very great 

 thicknesses ; 1 metre thickness is sufficient to render a flame almost 

 completely athermanous to rays coming from another flame of the 

 same nature. l — k e 



3. The formula y=a — - — r-r very well represents the intensity 



of the thermal radiation emitted by flames of any thickness e what- 

 ever, the transparency-coefficient being Jc =0-865, the thickness e 

 being expressed in centimetres, and a being a constant the value of 

 which depends on the nature of the flame. 



4. The absolute thermal emissive power of the white flames pro- 

 duced by illuminating- gas (that is to say, the intensity of the radia- 

 tion from such a flame possessing an infinite thickness, compared 

 with the intensity of the radiation emitted by lampblack at a tem- 

 perature equal to the mean temperature of the flame) is equal to 

 unity. 



The absolute thermal emissive power of the pale blue flames pro- 

 duced by Bunsen burners is represented by the fraction 0*3219 ; 

 that is, it is nearly one third of the emissive power of the white gas- 

 flames. 



5. The relative emissive power of a flame of a determined thick- 

 ness can be obtained by multiplying the ratio between the intensity 

 of its radiation and the maximum intensity (the intensity of the ra- 

 diation from the same flame if its thickness were infinite) by the 

 number which represents the absolute thermal emissive power of 

 that species of flames. 



A pale blue Bunsen-flame with a thickness of 4 millims. has its 

 emissive power expressed by the number 0-01744 ; that is to say, 



