EXPLOSIONS IN COAL MINES. 2 c 7 



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 lisht 

 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 with- 

 out 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 hav- 

 ing 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 conse- 

 quence been abandoned. But, be the cost of producing an equal quan- 

 tity 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 super- 

 sede the miner's lamp. Each man who works at the face must be pro- 

 vided 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 work- 

 ing of collieries, whether in the " long wall " system or on any modifi- 

 cation 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 por- 

 tion 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 inten- 

 sity of the transmitted is exactly equal to that of the reflected light. 

 By accurately measuring the distances, and applying the rule that the 



