393 



the cylinder of lime and the slit, causes two dark lines of remarkable 

 sharpness and fineness to show themselves in their stead ; but the 

 Drummond light requires, in order that the lines D should come out 

 in it dark, a salt-flame of lower temperature. The flame of alcohol 

 containing water is fitted for this, but the flame of Bunsen's gas-lamp 

 is not. With the latter the smallest mixture of common salt, as 

 soon as it makes itself generally perceptible, causes the bright lines 

 of sodium to show themselves." 



Now, when we heat a piece of ruby glass in the fire, we have an 

 analogous phenomenon. As long as the ruby glass is of a lower 

 temperature than the coals behind it, the light given out is of a red 

 description, because the ruby glass stops the green : the green here 

 is precisely analogous therefore to the line D, which is stopped by an 

 alcohol flame into which salt has been put. Should, however, the 

 ruby glass be of a much higher temperature than the coals behind it, 

 the greenish light which it radiates overpowers the red which it 

 transmits, so that the light which reaches the eye is green more than 

 red. This is precisely analogous to what is observed when a Bunsen's 

 gas-lamp with a little salt is placed before the Drummond light, when 

 the line D is no longer dark but bright. 



In fact, the law, " the absorption of a particle is equal to its radia- 

 tion, and that for every kind of light," only applies to the case where 

 the temperature of the particle is equal to that of the source of the 

 light which passes through the particle. If the temperature of the 

 source of light be greater, one quality of light will predominate ; if, 

 on the other hand, the temperature of the particle be greater, another 

 quality of light will predominate. 



III. " On the Luminous Discharge of Voltaic Batteries, when 

 examined in Carbonic Acid Vacua." By J. P. GASSIOT, 

 Esq., F.R.S. Received February 6, 1860. 



On the 24th of May, 1859 (Proceedings, May 26, 1859), I com- 

 municated to the Royal Society a short notice of my having obtained 

 the stratified discharge from a voltaic battery of 3520 elements 

 charged with rain-water ; and also with one of 400 elements charged 

 with nitric and sulphuric acids, each cell of both batteries being 

 insulated : I stated also that with the latter (as I had previ- 



