492 



Prof. F. Clowes, 

 the glass. 



[Feb. 2, 





FIG. 3. 



Flame. 



Window. 



Eye. 



SO mm 



-- 1 50 mm. 



the cap, and was corrected by multiplying it by the distance between 

 the eye and the cap (230 mm.), and dividing the product by the 

 distance between the eye and the glass (150 mm.). 



It may be mentioned that every reading was made in duplicate ; 

 most readings were repeated many times, and in many cases were 

 taken by more than one observer. Successive readings of the cap 

 produced by the standard hydrogen flame in the same percentage 

 of gas were frequently identical, and rarely differed by more than 

 1 mm. In the case of flames which could not be brought with 

 certainty to an invariable standard condition, the readings were 

 naturally less constant, but this arose from no fault in the method of 

 measurement adopted. 



The lamps used for testing should be kept clean. Small open- 

 ings protected by gauze must, if above, be especially kept free from 

 obstructive deposits of soot from the flame ; and, if below, must be 

 cleansed from films of oil, which may close the meshes of the gauze 

 entirely. 



It is noticed generally, that the caps produced by low percentages 

 of gas are pale and wanting in density and definition. As the per- 

 centage of gas increases, the cap gains in size, in density, and in defini- 

 tion, and wraps round the flame to its very base. The testing flame 

 also increases gradually in dimensions as the percentage of gas rises, 

 and in the presence of 3 per cent, of gas and upwards the testing 

 flame becomes more bulky, growing both in height and in girth. It 

 is therefore important to note that the 10-mm. hydrogen-flame was 

 adjusted in gas-free air, because its size did not materially alter until 

 over 3 per cent, of gas was present; the 15-mm. and 5-mm. flames, 



