262 LIGHT SCIENCE FOR LEISURE HOURS. 



Another peculiarity sometimes noticed when there 

 is a discharge of fire-damp is worth mentioning. It 

 happens, occasionally, that the light will be put out 

 owing to the absolute exclusion of air from the lamp. 

 This, however, can only happen when the gas issues 

 in so large a volume that the atmosphere of the pit 

 becomes irrespirable. 



With the exception of the one risk which we have 

 pointed out above, the Davy lamp may be said to be 

 absolutely safe. It is necessary, however, that caution 

 and intelligence should be exhibited in its use. On 

 this point Professor Tyndall remarks that unfortu- 

 nately the requisite intelligence is not often possessed 

 nor the requisite caution exercised by the miner, ' and 

 the consequence is that even with the safety-lamp, 

 explosions still occur.' And he suggests that it would 

 be well to exhibit to the miner in a series of experi- 

 ments the properties of the valuable instrument which 

 has been devised for his security. ' Mere advice will 

 not enforce caution,' he says ; ' but let the miner have 

 the physical image of what he is to expect clearly and 

 vividly before his mind, and he will find it a restraining 

 and monitory influence long after the effect of caution- 

 ing words has passed away.' 



A few words on the history of the invention may 

 be acceptable. Early in the present century a series of 

 terrible catastrophes in coal mines had excited the 

 sympathy of enlightened and humane persons through- 

 out the country. In the year 1813, a society was 

 formed at Sunderland to prevent accidents in coal 



