7. The Denayrouse Burner.—This burns nearly 300 liters of gas to produce 30 carcels, normally. As the photometric experiments are recent, let us suppose that it gives but 25 carcels. As 300 liters of gas represent an approximate expense of 10 centimes, we shall obtain the carcel at the price of 0.004, or at less than half a centime.
8. The Auer Burner.—This burns nearly 115 liters of gas to produce 5 carcels. The expense per carcel, with the cubic meter of gas at 30 centimes, is therefore 0.0069; say 0.7 of a centime.
Finally, in the United States, thanks to particularly favorable hydraulic installations, it is claimed that it is possible to produce acetylene at a very low price, say at 33 centimes per cubic meter. Under such conditions, the carcel would cost no more than 0.0025, say ¼ of a centime. It seems, however, that these are hypotheses as yet. If they chanced to be realized, it is certain that acetylene would be the light of the future; but those who are best informed in the matter assert that they never will be realized.
In order to establish still more accurately the net cost of each of these systems of lighting, it is necessary to take into account the wear of the mantles of the incandescent lamps and the carbons of the arc ones. As regards these latter, it is customary to estimate the wear of the carbons at 8 centimeters an hour.
As for the mantles, we shall base our calculations upon the data furnished by those interested; say 1,000 hours for the Edison lamp, 1,200 for the Auer burner and 400 hours for the Denayrouse burner. It must be remarked that in practice such duration generally drops to a half. The price of the mantles in these different systems is approximately 2.5 francs.
1. As the Edison 16 candle lamp gives 1.6 carcels and its filament burns 1,000 hours, the wear will increase the price of the carcel by 0.0015.
2. As the Auer burner gives 5 carcels and its mantle burns 1,200 hours, the wear will increase the price of the carcel by 0.0004.
3. As the Denayrouse burner gives 25 parcels, and its mantle burns but 400 hours, the wear will increase the price of the carcel by 0.0002.
Finally, if we compare the butterfly, Auer and Denayrouse burners with each other, in taking into account the cost of replacing the mantles of the two latter and the actuating of the Denayrouse burner, we find the following figures per carcel hour:
| Butterfly burner, | consumption | 0.04 |
| Auer burner, | consumption | 0.0069 |
| wear of mantle | 0.0004 | |
| Denayrouse burner, | consumption | 0.04 |
| wear of mantle | 0.0002 | |
| expense of motor | 0.0003 |