378 PEOFESSOE BUJfSEJs" AXD DE. H. E. EOSCOE’S PHOTO-CHEIHCAL EESEAECHES. 
degrees of luminosity and of varying form and colour. The principle upon which the 
lamp is constructed, is simply that the gas is allowed to issue under cu'cumstances in 
which it mixes with such a quantity of air, that the mixtui’e does not quite reach the 
point at which it is combustible without addition of oxygen. This condition is fulfilled 
by the following simple arrangement : — a, fig. 6 (Plate XVIII.}, is a common triangular 
burner, rising inside the cyhndrical cavity h to the surface of the cube cc. This cylin- 
drical opening, hb, which is 15 millims. in depth and 10 millims. in diameter, communi- 
cates with the outer an by fom* holes, dd^ 7 milhms. in diameter. After screwing on the 
tube which is 76 millims. long and 8-5 milhms. in diameter, the coal-gas is aUowed to 
issue from the burner a. On issuing, the gas causes a cm’rent of ah to enter the tube 
by the holes dd, and the mixtm-e burns at the upper end of the tube with an almost 
colourless flame perfectly free from smoke. On closing the openings dd, the original 
luminous smoky flame of coal-gas appears. 
In order to obtain a large and steady brightly-colom-ed flame, a small hoUow cylinder 
of porous coke, saturated with a concentrated solution of the salt which colours the 
flame, is placed on the upper end of the tube e. These small cyhnders of carbon are best 
prepared from the carbon employed as the negative pole in a zinc-carbon batteiy. In 
order to obtain the purest and brightest-coloured flame, it is necessary to be very careful 
to select pm’e salts, and to free the coke cylinder as much as possible from all foreign 
ingredients by ignition in a stream of chlorine, and boding out in aqua-regia. 
In the first place, the openings dd were closed, and the action of the luminous flame was 
found to be 21’5 in the minute. As soon as the holes dd were opened, and the iUuniina- 
ting power of the flame vanished, all perceptible chemical action also ceased. As the heat 
evolved in the non-luminous is greater than in the luminous flame, and as the tu'o flames 
only differ by the one containing incandescent particles of carbon which are wanting 
in the other, we must conclude that the chemical action is chiefly effected by the rays 
proceeding from the mcandescent carbon liberated dming the combustion. This carbon 
is produced from the elayl and ditetryl, the first of which varied in the samples of gas 
analysed from 4 to 5 per cent., and the second from 2 to 4 per cent. The exact photo- 
chemical measurements made with different portions of this gas appear, therefore, to 
point out that the amount of carbon set free in the combustion of coal-gas depends rather 
upon the dimensions and form of the flame than upon the small variations in the com- 
position of the gas. After the incandescent carbon, the carbonic oxide in the flame 
appears to effect the most chemical action, and although the small amount of 6 or 7 per 
cent, of carbonic oxide in the coal-gas is insuflicient to cause any perceptible action, still 
the flame of pure carbonic oxide produces much more action than that of hydrogen or 
light carbm-etted hydrogen. The smallest trace of foreign bodies in the flame can exert 
a great influence on the chemical action. The red, violet, yellow and green flames pro- 
duced by the chlorides of hthium, strontium, potassium, sodium and baiium, which can 
be seen to great advantage by the arrangement just described, give no more chemical 
action than the colourless flame. The fine green flame produced by chloride of copper, 
