UPON SOIME OF THE PHEXOIVIENA OE COMBUSTION. 
643 
EediTcing the means of these results, as before, to the maximum standard of 100, we 
hare the folloT\’ing numbers : — 
Third Series. 
Pressure of air 
in receiver in 
ins. of mercury. 
Mean illumi- 
nating power. 
Experimental. 
Mean illumi- 
nating power. 
Eedueecl. 
30-2 
83-4 
100-0 
29-2 
79-2 
930 
28-2 
747 
89 6 
27-2 
70-3 
84-3 
It is thus evident that the combustion of an amount of gas which will give a light 
equal to 100 candles when the barometer stands at 31 inches, will yield light equal only 
to 84’3 candles when the barometer falls to 28 inches. Such a variation in the lumi- 
nosity of gas-flames with the oscillations of the barometer will obviously elude the ordi- 
naiy modes of taking the illuminating power of gas, inasmuch as the standard light with 
which the gas is compared is also subject to the same influence. Still, although the rela- 
tive light of gas as compared with candles may remain nearly or quite unaltered, yet its 
absolute illuminating value depends greatly upon the height of the barometer at the 
place where it is burnt. Thus a quantity of coal-gas which in London would yield a 
light equal to 100 candles would, if burnt in Munich, give an illuminating effect equal 
to little more than 91 candles ; whilst if used to light the city of Mexico, its luminosity 
would be reduced to 61-5 candles. These numbers are independent of the change of 
volume by reduced pressui’e. If equal volumes of the same sample of coal-gas were 
consumed in London and Mexico, the illuminating effects would be as 100 : 46*2, the 
temperature being the same in both cases. 
An inspection of the above three series of observations, reveals the fact that the rare- 
faction of air, fi’om atmospheric pressure do^vnwards, produces a uniform diminution of 
light until the pressure is reduced to about 14 inches of mercury, below which the dimi- 
nution of illuminating powder proceeds at a less rapid rate. This uniformity of relation 
between pressure and luminosity will be more clearly seen from Plate XIX. diagrams 1 
and 2, in which the luminosity is represented by the ordinates, and the pressure by the 
abscissee measured from the origin B. If therefore the luminosity were simply propor- 
tional to the pressure, the curve of luminosity would coincide with the diagonal drawn 
from A to B in diagram No 1. Inasmuch, however, as the diminution of light is more 
rapid than the diminution of pressure, the lines A C and A D, representing the experi- 
mental results of the first and second series of observations, fall between this diagonal 
and the ordinate corresponding to the point A. Diagram No. 2 shows the results of the 
third series of observations : in order to render it as open as possible, only that portion 
of the square is given through which the experimental line AB passes. The line. AC 
in diagram No. 1 represents the average results of the first series of observations, whilst 
A D indicates those of the second series. It will be seen, from an inspection of both 
4 s 
MDCCCLXI. 
