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COMPRESSED GASES 89 to BaO, and the oxygen given off. The process thus uses up no chemicals whatever, and becomes simply mechanical. The immediate effect was to reduce the retail price of oxygen to 4:d. per cubic foot, and in large quantities to much less. Erin's gas is always perfectly dry, and free from chlorine ; and when also free from nitrogen, appears to me superior to average chlorate gas, though not to gas properly washed and dried (see p. 78). At first, however, the quantity of nitrogen left in it varied materially, and on several occasions I have been greatly disappointed with the light when employing this gas for microscopic or physical experiments. So much was this the case, that some London opticians have turned again to chlorate gas, and gone to the extra expense of pumping that at the lowered rate. I believe, however, that all the practical difficulties have now been overcome by the Brin Company, and the more recent cylinders I have had of their gas have been all that could be desired. 48. Using Condensed Gases.—With a good regulator screwed upon the cylinder, this is used precisely as a gas-bag. The regulator may be set at any given pressure in reason : I recommend 9 inches for ordinary lantern work, and 14 inches for high-power work. It has already been remarked that with blow-through jets no regulator is absolutely required, provided no taps are ever turned off, and all the regulation be done at the cylinder valve. But with the regulator the cylinder may be turned on, and the jet regulated by its taps as usual: also the jet may be turned completely off if needed. At the same time it is not wise to subject the mechanism to the immense pressure for longer than neces- sary ; therefore if the light is wanted off for more than ten minutes the cylinder-valve should be turned off, and it is well not to turn it on till just before the jet can be attended to. As a rule no further attention will be required, the pressure being maintained much more uniform than from bags; and it will be kept up till the gas left in the cylinder is no longer