THE BOOK OF THE LANTERN. 



unequal expansion which it undergoes. The shield pre- 

 vents this by confining the heat within a narrow area 

 around it, at the same time keeping the lantern itself com- 

 paratively cool. Mr. Wood tells me that there is an 

 undoubted gain of light from this conservation of heat in 

 and around the lime cylinder. The shield, moreover, does 

 much to obviate a by no means uncommon accident, 

 namely, the fracture of the valuable condenser itself from 

 a flame deflected towards it from a cracked or much-pitted 

 lime cylinder. 



The most general method of storing the gases required 

 for the lime -light is by means of bags, which should be of 

 the best quality. It is necessary to caution the beginner 

 on this point, for I have known cases where a perfectly 

 new bag, fresh from the maker, has been found, on trial, 

 to leak badly. I cannot say that such an unpleasant cir- 

 cumstance has happened to myself, for I go to a maker 

 upon whom I can rely. It may possibly have been the 

 fault of the operator himself, who did not take the pre- 

 caution to warm the bag before use. For these bags in 

 winter time when of course they are most generally used 

 get stiff and hard, and if not of good quality are apt 

 to crack, unless carefully warmed before use. 



The best bags are made of unvulcanised rubber, covered 

 on the outer side with twill, and on the inner side with coarse 

 canvas. This rough canvas serves a double purpose it 

 gives strength to the bag and also prevents the inner sides 

 sticking together when the bag is empty. After a bag has 

 been in use some time, the presence of this canvas makes 

 itself evident by a quantity of fibrous matter which 



