880 PEOrESSOE BTJISrSEN AJS’D DE. H. E. EOSCOE’S PHOTO-CHE^nCAL EESEAECHES. 
I. COMPAEATIVE AND ABSOLUTE jMEASUEEISIENT OP THE CHE:\nCAL EATS. 
In all photometric measurements, whether made with the eye, or by means of thermic, 
thermo-electric, or chemical agencies, the great difficulty has been to obtain a convenient 
constant source of light. 
The flames of ordinary candles or lamps are subject to variation in them optical and 
chemical illuminating power, such as to prevent their being employed when any great 
degree of accuracy is required. The intensity of the light evolved by a wire heated to 
whiteness between the poles of a constant battery is also extremely variable, as an 
imperceptible alteration in the quantity of circulating electricity effects a considerable 
change in the luminosity of the incandescent wire. These difficulties are, however, 
overcome when a flame is employed which is fed by a cuiTent of gas issuing with unifonn 
velocity. Olefiant gas cannot be advantageously used for the production of such normal 
flames, not only on account of the difl&culty of preparing large quantities of this gas in 
a state of purity, but also because on combustion it suffers very comphcated decom- 
positions. Carbonic oxide, on the other hand, burns without ehmination of caihon, 
does not undergo other secondary decompositions,*and forms one product of combustion, 
namely, carbonic acid. In other respects too, especially on account of the considerable 
chemical action which it effects, as also from the ease with which it can be prepared in 
a state of purity from the salts of formic acid, the flame of carbonic oxide is particularly 
well adapted for absolute photo-chemical measurements. 
The amount of light radiating from a flame does not merely depend on the mass and 
constitution of the illuminating material, but is often even more influenced by the 
particular circumstances under which the combustion is effected. e ha^ e therefore 
been obliged to determine, more accurately than we had preriously done*, the condi- 
tions most favourable to the constant evolution of light from flames of various kinds. 
The flames formed by the combustion in free air of gases issuing from small openings 
under a pressure of even only a few millimetres of water, are subject to such changes in 
form and temperature, arising from the production of lateral currents of air, that they 
cannot be employed as constant sources of light. When, however, the pressure under 
which the gas issues differs but very slightly from that of the atmosphere, the flame 
assumes a perfectly constant form. In the fdllovring experiments the rate at which 
the gas issued from the burner during the combustion of the caihonic oxide, which 
formed our constant source of light, was only 129*9 millimetres in the second. By 
means of the formula for the rate of issue of gases, not regarding the friction, which in 
this case is very small, we have calculated the amount of the difference of pressm'e 
which forces the gas out, and found it to be equal to a column of water of 0*001 milli- 
metre in height. When the gas issues under so slight a pressure, and burns from an 
opening of several millimetres in diameter, placed in the blackened tin box which we 
have formerly described f, the flame assumes the form of a very obtuse cone and bm*ns 
perfectly steadily. 
* PhilosopHcal Transactions, 1857, p. 374. 
t Ibid. p. 375. 
