278 



SCIENTIFIC AMUSEMENTS. 



iionluminous flame, like that of a properly arranged Bunsen 

 burner. 



Expt. 325. The safety lamp. Fig. 131 represents a form of 

 safety lamp, based on much the same principle, 

 consisting of a small oil lamp surmounted by a 

 wire gauze cage. If the lamp be lighted and 

 the cage fixed on, the lamp may be held in a 

 mixture of coal gas and air (such as that issu- 

 ing from a large unlighted Bunsen burner) 

 without firing the mixture, although a blue 

 flame is formed inside the cage if sufficient 

 inflammable gas passes through the meshes to 

 burn inside. In actual use underground, the 

 presence of a dangerous amount of firedamp in 

 the atmosphere is indicated by a peculiar " cap " 

 or fiery appearance inside the lamp, due to 

 this action ; in such a case the lamp ought to be 

 extinguished, as, if kept alight, the gauze is very 

 apt to be heated red hot, when it might permit 

 the flame to pass through, on account of the cool- 

 ing action being no longer sufficiently exerted. 



Any sudden shock or impulse given to the 

 air (as by firing a blast, a sudden fall of 

 rock, &c.) is liable to drive the flame of 

 even the best safety lamps through the gauze for an instant, the 

 cooling action being in such a case not sufficiently rapid to chill the 

 flame down below the point of ignition, or temperature sufficient 

 to cause the chemical action of burning to commence. Should 

 this occur, of course any explosive atmosphere outside the lamp 

 would be forthwith fired ; hence, although safety lamps have 

 proved a great boon to colliers by diminishing the risk of explo- 

 sions, they are by no means perfect safeguards, especially when 

 used carelessly and incautiously. 



Expt. 326. Illustration of bad conducting Power of Water. 

 Water possesses only an extremely low power of conducting 

 heat, although from its fluidity and mobility convection takes 

 place very readily, not only with pure water, but also with all 

 aqueous fluids and solutions, unless rendered extremely viscid (like 

 treacle), by the presence of very large quantities of dissolved matter, 

 Tie a piece of lead or brass to a small lump of ice, or put both 

 into a small bag of cambric or netting, and place them in a large 

 test-tube filled with ice cold water, the piece of metal being 

 sufficiently heavy to cause both to sink to the bottom. Now heat 

 the upper part of the test-tube by means of a lamp, holding the 



Fig. 131. Safety 

 Lamp. 



