THE BOOK OF THE LANTERN. 59 



comes out of the jet, and which I have been gravely told 

 by an ignorant operator is a deposit left by the oxygen 

 gas! 



The bag is furnished with a brass stop-cock, which is 

 apt to get so stiff as to be very difficult to turn. For this 

 reason the screw holding the plug of the tap should be 

 undone frequently, and the plug touched with a little oil, 

 vaseline, or tallow. This stiffness is due to free chlorine, 

 which, as is explained in the chapter on oxygen making, 

 is often present in that gas. The purchaser of a bag is of 

 course to a great extent at the mercy of the trader, and 

 many inferior bags are sold to the unwary. But a few 

 inquiries among those who know what a good bag should 

 consist of will soon inform the buyer where he can pur- 

 chase one which is reliable in quality. Certainly too rigid 

 an economy should not be exercised in this particular part 

 of the lantern equipment. 



The gas-bags are made wedge-shaped so that they 

 can be placed between sloping pressure boards fur- 

 nished with weights. I have heard of careless operators 

 who are content to trust to luck for finding suitable 

 boards for their gas-bag, or bags, when they arrive at the 

 scene of operations. This is, of course, a most repre- 

 hensible proceeding. But should an exhibitor be so placed 

 that he cannot obtain boards, a blackboard, such as can 

 be found in any schoolroom, can be used for the purpose, 

 provided that it is fastened by staples to the floor. It 

 must also have a shelf at the other end, on its upper side, 

 against which the weights can rest. This is but a make- 

 shift, and one which should only be resorted to in an 



