This points to the inquiry, What is the true source of light? From what materials, as matter of principle, and apart from any question of the state of the science of illumination at the moment, is artificial light more certainly to be obtained?
To that question the reply is simple. We know, as matter of chemistry, what kind of combustion produces the greatest amount of light, as we also know what produces the greatest amount of heat. The two are by no means identical. Light can not be produced without the liberation of heat. On the other hand, a very high degree of heat can be developed when, little or no light is produced. As matter of principle, this is the key to the question now to be reviewed.
We need not at the moment step aside to inquire into the future of the electric light. As to the cost at. which that elegant source of concentrated brilliancy may be maintained, we are in the way of having experimental proof. The first great trial in London, that of the Jablochkoff candles at Billingsgate Market, has proved a failure, as regards both the quantity and the quality of the light produced, as well as with reference to the cost of production, and has in consequence been abandoned. But, be the cost of producing an equal quantity of light by the new or the old fashioned process of combustion the greater, the former is out of the question as far as coal mines are concerned. A brilliant light at the bottom of the shaft would of course be a great desideratum. But no one who has studied the plan of the workings of a coal mine can fail to be aware that nothing will supersede the miner's lamp. Each man who works at the face must be provided with his own light; and no general illumination, were such possible, would make up for the want of this. In vast underground caverns, such as that of the Peak, in Derbyshire, or such as those of some of the Cheshire salt mines, a brilliant and concentrated light may, no doubt, be extremely effective. But in speaking of the working of collieries, whether in the "long wall" system or on any modification of the "pillar and stall," we must look to such a lamp as each miner can carry for himself.
In speaking of illumination, we are as yet without any unit of light. Our measurements in this respect are made pretty much by rule of thumb. The sperm candle, burning or supposed to burn at the rate of one hundred and twenty grammes per hour, is our nominal unit. In ascertaining the illuminative power of gas, two of these candles are used by way of measure. But there is no check as to the accuracy of their consumption. The use of a screen made diaphanous in one portion by a little grease enables the analyst to form a very accurate appreciation of the illuminative power of two lights. The screen is placed between the two, and moved backward or forward until the spot caused by the grease vanishes, which is the case when the intensity of the transmitted is exactly equal to that of the reflected light. By accurately measuring the distances, and applying the rule that the
