THE COMBUSTION OF GAS FOB ECONOMIC PUBPOSES. 
481 
the stroke, and it generally ends with a good deal of unutilized pressure. In one case I 
find that the indicator recorded an initial pressure of 75 lbs. on the inch at the moment 
of explosion, and a final pressure of 25 lbs. The loss of power in this case must have 
been considerable, for not only is there the loss of the difference (12-5 lbs.) between the 
calculated pressure 37-5 lbs. (75-^-2), and the real (25), but there is also the total loss of 
the unavailable final pressure. A part of this loss is no doubt due to leakage, and to the 
cooling effect of the walls of the cylinder, for the temperature has been observed to fall 
from 2474° Fahr. at the moment of explosion to 1438° at the end of the stroke—the cal¬ 
culated temperature being 2156° ; indeed the management of the temperature is one of 
the difficulties of the engine, for the cylinder has to be cooled by a stream of water. 
Improvements will no doubt be made in the construction of the engines, and especially 
in the utilization of the residual power, and this must be done by shutting off the valve 
and firing the gas earlier in the stroke. This has already been done to some extent in 
America with engines of half-horse power, as with cylinders of 4f inch diameter by 8f 
inch stroke ; and this with 185 revolutions or 370 explosions in a minute raises 16,280 lbs. 
one foot high in a minute. In France and in this country much larger engines are 
made, as from 1 to 3 horse power. 
The quantity of gas used in the working of the engine is rather variable. In the 
American engine, already alluded to, it took 105 cubic feet of gas an hour to work an 
engine of half-horse power, and a one-horse engine in London takes about 185 cubic feet 
of London gas—say it is 200 cubic feet—per horse power. This is 1,980,000 lbs. a foot 
high ; whereas the theoretical power of 200 feet is more than 100 millions of pounds. 
The advantages of the engine are very great, for it takes up but little room, it is very 
clean, it works with great regularity, it requires little or no attention, and it costs nothing 
for fuel when it is not at work. 
One thing I ought to mention in speaking of the explosive poAver of mixed gas, and 
that is the effect of using mixtures in improper proportions. Sir Humphrey Davy found, 
in his experiments with marsh gas, that there was but one proportion of air and gas 
which gave the maximum effect, and that was a mixture of 1 of gas and 7*5 of air (theo¬ 
retically it should be i to 9’5). When the proportions are reduced in either direction the 
mixture becomes less and less explosive, until with 1 gas and 15 air, or with equal 
volumes of gas and air, the mixture ceases to explode. 
In the case of coal gas, although the theoretical proportions for London gas are 1 of 
common gas* to 5‘6 of air, and 1 of cannel gas to 7'4 of air, yet the best results are ob¬ 
tained with 1 of the former to 8 of air, and 1 of the latter to 11. On either side of this 
proportion the mixture rapidly becomes less and less explosive. 
The effect of mixing other gases with explosive mixtures has been w'ell studied by 
Davy and others: taking, for example, an explosive mixture of 2 volumes of hydrogen 
and 1 of oxygen, it is found that 1 of nitrogen to 6 of the gas, or 1 of carbonic acid to 
7 of it, will stop its explosion. 
Lastly, the temperature at which these gases are fixed is a matter of considerable im¬ 
portance. Davy found that he could not set fire to marsh gas (the firedamp), or to an 
explosive mixture of it with air, by using the strongest heat of glowing charcoal. He 
even blew a mixture of the gas upon glowing charcoal until he got it at a maximum 
heat without firing it; nor can it be fired by the sparks from flint and steel. Not so, 
however, with hydrogen, or olefiant gas or carbonic oxide, all of which are fired by the 
sparks and by glowing charcoal—perhaps the igniting temperature is about 3900° Fahr.; 
and the vapour of bisulphide of carbon is fired at as low a temperature as 300° Fahr. 
* Average Composition 
ofJjondon G-as 'by Vohmi'’. 
Common Gas. 
Cauuel Gas. 
Hydrogen. 
Light carburetted hydrogen . 
. 39’5 . . . 
. . . 60-0 
Olefiant, etc. 
. 3-8 .. . 
. . . 13-0 
Carbonic Oxide. 
. 7-5 .. ; 
... 6-8 
Carbonic acid.: 
. 0-7 .. . 
Aqueous vapour. 
. 2-0 .. . 
Nitrogen. 
100-0 
100-0 
