458 Prof. V. B. Lewes. The Cause of Luminosity [Mar. 21, 



Illuminating value 

 Analysis of mixture. per 5 cub. ft. 



t \ 



Hydrogen. Acetylene. Argand. No. 4 Bray. 



92 8 Not measurable 



91 9 Not measurable 



88-5 14-5 4-1 17 



so that luminosity would be destroyed in the argand by dilution with 

 about 90 per cent, hydrogen, and in the No. 4 Bray with about 88 

 per cent. 



The next point to be determined was whether the destruction of 

 luminosity in the diluted acetylene flame was in reality due to dilu- 

 tion rendering it necessary to employ a higher temperature for the 

 decomposition of the acetylene, or to other causes. 



In order to do this, a tube made of specially infusible glass 4 mm. 

 in diameter was taken, and the Le Chatelier thermo-couple was 

 fitted into it in the same way as before, used with the platinum tube, 

 and all air having been rinsed out by a current of the mixture to be 

 experimented with, the gas was allowed to pass at a steady rate of 

 flow through the tube, the point at which the thermo-couple was 

 situated being steadily heated by the Fletcher blowpipe, whilst the 

 temperature recorded on the scale was noted the moment that incan- 

 descent liberation of carbon commenced. 



Percentage composition of gas. Temperature necessary 



( A ^ to cause deposition of 



Acetylene. Hydrogen. carbon with luminosity. 



100 780 C. 



90 10 896 



80 20 1000 



It was found impossible to obtain a glass tube which would stand 

 temperatures higher than this; but on plotting out the points so 

 obtained, and which give a fairly straight line, it is seen that even if 

 the increase in temperature only continues for increased dilution in 

 the same ratio as shown in the experimental determinations, which 

 is extremely unlikely, the reason of the destruction of luminosity in 

 highly-diluted hydrocarbon gases is at once explained, as an increase 

 of each 10 per cent, in the dilution would necessitate an increase of 

 100 C. in the temperature of the flame, and with 90 per cent, dilu- 

 lution a temperature of over 1700 C. would be required to bring 

 about decomposition. 



My reason for believing that it is highly improbable that; when 

 dilution is great it only requires the same increment in temperature 

 to bring about decomposition as when the dilution is small, is that in 

 all the work I have done on the effect of diluents upon luminosity, 

 and also in Professor Percy Frankland's researches upon the same 



