128 Mr. E. H. Liveing on a new Instrument for the 



and offering equal resistances and cooling-surfaces, become 

 equally heated on turning the machine; one of these spi- 

 rals is enclosed in a small tube having a glass end and 

 containing pure air ; the other is exposed within a small 

 cylinder of wire gauze (also with a glass end) to whatever 

 gaseous mixture is drawn within the instrument for ex- 

 amination. 



So long as the air examined is free from combustible gas or 

 matter, the two spirals glow with an equal brilliancy; but if 

 the air contain any amount above ^Jq of its volume of CH 4 

 the exposed spiral increases in brilliancy, and the more so the 

 nearer the percentage approaches the explosive proportion; 

 and this difference of brilliancy becomes a means of deter- 

 mining the percentage of gas present. 



To measure the difference of brilliancy there is a simple 

 form of photometer, consisting of a small wedge-shaped screen 

 C, the two surfaces of which are covered with paper and 

 illuminated, the one by the covered spiral, the other by the 

 exposed one. These two surfaces are viewed through the 

 small side tube G, which, together with the screen and outer 

 tube K, can be moved towards the covered spiral until equal 

 illumination of the two surfaces of the screen is attained in 

 the particular gaseous mixture under examination. An empi- 

 rical scaleis then read off, which gives at once the percentage 

 of gas present. 



The following table shows the difference of brilliancy due 

 to stated percentages of CH 4 ; but the amount much depends 

 on the size of the wires, proportion of the spiral, and other 

 conditions, and can only be determined by actual trial in 

 known mixtures. It is a point worthy of note that the rate at 

 which the machine is turned does not seem to have any 

 sensible effect on the ratio of brilliancy of the spirals so long 

 as the exposed one is heated to the igniting-point of CH 4 . 



Table of observed Illuminating-power of the Spirals in different 

 percentages of gas. 



Per cent. CH 4 . 



Relative illuminating-power of spirals. 



Covered spiral. 



Exposed spiral. 



Pure air. 



% per cent. CH 4 . 



\ „ 



1 » 



2 „ JJ 



3 „ 



4 „ 





1 



1-24 



1-65 



2'78 



5-1 



22 



64 



