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64 THE BOOK OF THE LANTERN. quired to be shown. Using a disc of from 15 to 18 feet in diameter the exhibitor will do well to commence when the bags are full with two half-hundred weights on his pressure boards. But when the gas has been so much used that the upper board gets nearly horizontal the pressure will be lessened, and the light will suffer to some extent. When this happens, the experienced operator will place another half-hundredweight in position, and the increased bright- ness of the picture will quickly show the advantage of so doing. When the gas or gases are drawn direct from steel cylinders, or bottles, if a double or triple lantern is in use, some form of regulator must be employed. The first introduced, and perhaps the most perfect, is that patented by Messrs. Oakley & Beard. Before proceeding to describe this important new de- parture in lantern working, it may be as well to point out one or two difficulties which are incidental to the ordinary method of storing the gases required in india-rubber bags. So far as the writer knows, one of these difficulties has never been recognised in print. This difficulty is com- prised in the fact that any kind of gas, if kept in an india- rubber bag, quickly deteriorates. By the phenomenon known by the term endosmose, two gases separated by a porous diaphragm will effect a mutual exchange. Take the case of an ordinary india-rubber tube used.for a table lamp or gas stove. How quickly it begins to smell. This is nothing else but the gas escaping through the india-rubber, and carrying on an exchange with that other gas—the air—which is outside. It would be interesting