PROGRESS IN ARTIFICIAL LIGHTING. 613 
of us remember as of yesterday diffused impurities in the air far greater in pro- 
portion to the light they afforded than any modern form of Hght. The last fifty 
years have witnessed vast improvements in the wicks and in the materials of 
candles, so that their smokiness has been gradually much reduced. Indeed, as 
regards the contamination of the air of a room, it may be accepted as an axiom 
that the more imperfect the combustion in any source of artificial light, the more 
deleterious the effect on the air of the room. We complain of gas, but if we 
were satisfied with the same amount of light in the case of gas which we obtain 
with candles or lamps, we should not find the vitiation of the air -more conven- 
ient with one than the other. 
An experiment mentioned by Mr. Clegg in his treatise on the manufacture 
of gas may be mentioned here in illustration of this. The flames of several com- 
bustible bodies that give an amount of light equal to it, were burned separately 
in given quantities of atmospheric air, and the times were noted at which the 
flames were extinguished by the contamination of the air. The following were 
the results : 
Minutes. 
Colza oil was extinguished in 71 
Olive oil was extinguished in 72 
Russian tallow was extinguished in 75 
Sperm oil was extinguished in 76 
Stearic acid was extinguished in 77 
Wax candles was extinguished in . . 78 
Spermaceti candles was extinguished in 83 
Coal gas (13 candles) was extinguished in . . . . 98 
Cannel gas (28 candles) was extinguished in . . . .152 
The preceding numbers may be taken to indicate the comparative salubrity 
of the several illuminating materials, from which it appears that the atmosphere 
of a confined room lighted by cannel gas would support Hfe twice as long as the 
atmosphere of the same room lighted equally with tallow candles. Nor does the 
complaint that is frequently made of the heat of rooms heated by gas afford much 
better foundation for an objection to gas lighting than its assumed insalubrity. 
The fact may be true that a room lighted by gas is hotter than when lighted by 
candles, but the cause is to be attributed, not to the greater heat-giving power 
of the gas, but to the greater illumination when gas is employed. If persons 
would be satisfied with the same dim light to which they are accustomed when 
burning candles, or if they would increase the number of the latter so as to equal 
the light of the gas flame, the heat given out would be found less when burning 
gas than when burning lamps or candles. It has, indeed, been proved by exper- 
iment that the combustion of colza oil produces nearly twice as much heat as 
the flame of cannel gas of the same standard of luminosity, and that in compari- 
son with ordinary 13 sperm-candle gas the proportionate amounts of heat are as 
78 to 68. A room lighted by a large moderator lamp burning colza oil is per- 
