PROFESSOE BUNSEN AND DR. H. E. ROSCOE’S PHOTO-CHEMICAU RESEARCHES. 3G5 
Scott’s burner at which an alteration in the dimensions of the flame produced the least 
variation in the luminous intensity. In the following experiments, conducted for this 
purpose, the intensity of the light from the most luminous parts of flames of different 
dimensions was measured by allowing the various amounts of light to fall through the 
same opening upon the sensitive gas in the instrument. The first horizontal division in 
the following Table contains the heights of the various flames measured by the millimetre 
scale on the chimney ; the remaining divisions give the amount of chemical action pro- 
duced by radiation from equal surfaces of the various flames. 
Series of Experiments V. 
Height of the flame. 
82 millims. 
95 millims. 
110 millims. 
133 millims. 
r 
21-0 
18-77 
13-84 
9*0 
Observed intensity] 
22-0 
19*17 
17*31 
9*0 
of the chemical rays 1 
19-0 
17-61 
16-39 
9*0 
1 
21-0 
19-23 
17*20 
9*0 
Mean intensity 
20-75 
18-69 
16-78 
9*0 
These experiments give the following variation in the intensity of the chemical rays 
for alteration in the heights of different-sized flames : — 
Height of flame. 
Intensity of 
chemical rays. 
Difference of 
height. 
Difference of 
intensity. 
130 mm. 
110 „ 
95 „ 
82 „ 
9*00 
16-78 
18-69 
20-75 
20-0 mm. 
15-0 „ 
13-0 „ 
7-78 
1*91 
2-06 
Hence it is seen that when the flame is from 95 to 110 millims. in height, alterations 
in the size of the luminous cone are accompanied by the smallest alterations in the 
intensity of the chemical rays. For this reason, in all the experiments in which the 
annular burner was employed, we placed the flame at a height of about 100 millimetres. 
As the limit of height within which the flame could be kept constant was about 6 mil- 
limetres, and as accorchng to the above experiments a variation of 15 millims. in the 
height of the flame causes an alteration of of the total intensity, the errors in the 
determinations arising from variations in the size of the flame could not amount to more 
than 4 per cent., supposing that no other errors occurred from differences in the compo- 
sition of the coal-gas employed. The accurate agreement of a large number of photo- 
chemical measurements made at different times with the Heidelberg coal-gas, for the pre- 
paration of which the process remained unaltered during the period of our experiments, 
proved, as we shall show, that no appreciable difference in the intensity of the chemical 
rays occurred in the flame of coal-gas prepared at various times. The next point to 
determine was the influence which the strength of the electrolysed hydrochloric acid, as 
well as the intensity of the current employed to decompose it, exerted upon the compo- 
sition of the sensitive gas ; and the length of time which is necessary to lead the gas 
