﻿and its relations with the Pressure. 113 



proportion as luminous sources at higher temperatures are used 

 to produce them. This is a cardinal observation due to M. 

 Mascart. Thus the number of lines increases in the proportion 

 in which the temperature rises in the flames which produce them ; 

 and when the temperature attains a certain intensity, these lines 

 merge into one another and give a continuous spectrum. The 

 flame then necessarily becomes white, brilliant, and luminous. 



A fact of the same kind is produced in Professor Frankland's 

 experiment. The lines increase in number and in intensity in 

 the hydrogen flame in proportion as the pressure on the explosive 

 mixture within and without the blowpipe itself increases. What 

 more rational conclusion can be drawn than that the tempera- 

 ture itself increases in the flame in proportion as the pressure in- 

 creases ? This is a prime fact, the demonstration of which may 

 seem sufficient; but it is full of consequences so important that 

 direct verifications should still be demanded. I shall revert sub- 

 sequently to the consequences and the methods of verification 

 which I think of using; but I desire to show at once that these 

 considerations, deduced from spectrum-analysis, will explain the 

 fact of the great illuminating power of arseniuretted hydrogen — 

 a power that Davy's theory, which I think is incomplete from 

 this point of view, can only explain by the supposed presence of 

 a solid body in the flame. It is sufficiently evident that gases 

 in burning give lines. If these lines are brilliant and numerous 

 for reasons depending on the special nature of the substances 

 observed, it is clear that the flame of these gases will be brilliant 

 and the more luminous the greater the difference in the refran- 

 gibility of the lines these spectra contain. Here we have to do 

 with a phenomenon belonging to arsenic in vapour contained in 

 the flame of arseniuretted hydrogen ; and in the explanation of 

 such a fact it seems to me useless to bring in the consideration 

 of densities, invalidated moreover by the objection urged by 

 Professor Frankland himself in the case of the flame of phos- 

 phorus burning in chlorine. 



Thus the illuminating power of an entirely gaseous flame is a 

 specific property connected with the production of lines furnished 

 by the substances it contains ; it is as inexplicable as the specific 

 properties of the bodies themselves — the den sit} 7 , colour, &c. 

 Professor Frankland's idea, moreover, relative to the production 

 in ordinary flames of very dense carburetted hydrogen seems to 

 me difficult to rest on experiment. We know, in fact, that all 

 these carburetted hydrogens decompose at the lowest tempera- 

 tures into hydrogen and carbon — the latter hydrogenized,itistrue, 

 but opaque*. I think, then, that Davy's theory remains intact. 



* I have proved (Lecons sur la Dissociation, p. 317, Lecons de la Societe 

 Chimique. Paris, Hachette, 1866) that in strongly heated carbonic oxide 



Phil. Mag. S, 4. Vol. 37. No. 247. Feb. 1869. I 



