THE LIME-LIGHT 43 



gum dissolved in naphtha. Having procured a piece of thin 

 waterproof the kind of which nursing aprons are made and 

 cut a piece to cover the leaky place with a good margin, the 

 solution is smeared all over one side of it, and over the space 

 on the bag it is to cover. It must be left for half an hour to 

 two hours, until it has become very sticky or ' tacky,' when 

 the patch is applied, all air worked smoothly out, and the 

 patch then left under a heavy weight, with a soft cloth 

 collected into a pad between, to equalise and apply the 

 pressure. The leak can generally be found by the smell, if 

 the bag is filled with house-gas and pressure applied. 



Definite leaks can be stopped in this way, especially such 

 as may be caused by mischievous boys sticking pins into a 

 bag ; but when they get numerous it is a sign that the india- 

 rubber is worn out generally. 



The cubic content of a wedge-shaped bag is of course 

 found by multiplying its three dimensions and taking half the 

 product ; but the bulging of the bag when full, will add from 

 one to two feet more. A size 3x2x2 feet will keep all 

 ordinary jets going in a bi-unial lantern for two hours, but is 

 not sufficient for the powerful jets presently described. To 

 be certain there is plenty of gas is a very great comfort ; and 

 to be uncertain, very much the other thing. 



Gas should never be kept in a bag any length of time. 

 Oxygen acts rapidly upon the material ; but independently of 

 this, no gas can remain pure, owing to that wonderful process 

 of diffusion through the material which physicists call osmosis. 

 A bag filled with perfectly pure oxygen, will contain a portion 

 of air in some hours' time, and any bag tight in the morning 

 is perceptibly slackened by night. Supposing gas to be left 

 over one night, to any amount, there is no reason, if con- 

 venient, why it should not be kept, and merely filled up for 

 the next night ; but beyond this gas should never be kept, or 

 the light will be perceptibly affected. 



keep best in a moderate temperature, both cold 



