'2'1 1 Influence of Pressure on the Spectra of Flames. [Feb. 19, 



at the respective pressures, are approximately proportional to the 

 squares of the pressures. 



This may be taken to indicate that the brightness of the continuous 

 spectrum depends mainly on the mutual action of the molecules of 

 gas. 



A series of similar observations on hydrogen burning in oxygen 

 gave somewhat different results, tabulated below : 



The flame was brighter than thafc of oxygen burning in hydrogen 

 at ordinary pressure, but the rate of increase with increased pressure 

 was not so rapid as in the former case. It seems as if the continuous 

 spectrum were made up of two parts, one varying as the square of 

 the pressure, and another according to some other law. The flame is 

 evidently not the same in the two cases. The products of combustion 

 derived from the small quantity of air are different, and also the 

 hydrogen jet always showed the presence of sodinm, sometimes 

 calcium. The appearance of the flame was also different; the hydro- 

 gen jet being faintly visible and yellowish in the elongated part, 

 whereas the light from the oxygen jet was concentrated near the 

 base, the point being invisible. The measures of which the means are 

 tabulated above were also less concordant than the corresponding 

 measures for the oxygen jet. We were unable to carry our measures 

 beyond a pressure of 95 Ibs., because at higher pressures a cloud was 

 formed in the apparatus which prevented our seeing the flar 

 directly. We hope to prosecute these measures with flames of oth< 

 gases, and, if possible, at higher pressures. 



The conclusions to which our experiments have led seem incoi 

 sistent with those which have been drawn from Pliicker and Hittoi 

 well-known observations on the widening of the hydrogen lines in 

 vacuous tubes with a residue of hydrogen when that residue increases. 

 That the widening of the lines in a Pliicker's tube results from 

 increasing the density of the residue of hydrogen in the tube cannot 

 be gainsaid, but we are wholly ignorant of the mechanism by which 

 the gas is lighted up by the electric discharge. It is sometimes 

 assumed, but without any sufficient reason, that the energy of the 

 electric current is first converted into heat, and then in turn into 



