The two gases, oxygen and hydrogen, required for the production of the lime-light need now some separate notice, as it often occurs that those who use these gases for dissolving-view exhibitions have to prepare the one--sometimes both--for themselves.
Oxygen gas is a simple elementary body, and is most extensively diffused throughout our earth. It is one of the constituents of the atmosphere surrounding our globe, and also of water, as well as of nearly all the substances known as acids. It has never yet been met in its separate or uncombined state, and therefore it can only be obtained by decomposing some substance of which it is a constituent.
The material most convenient for obtaining oxygen gas for lime-light purposes is chlorate of potassa. Two parts of this material (by weight) is mixed with one part of powered black oxide of manganese (this is known as ‘oxygen mixture’). The oxide of manganese does not supply any of the oxygen, but its presence facilitates the decomposition of the chlorate of potassa. The apparatus used for generating the gas is shown at Fig. 9. It consists of a copper or iron vessel (A), forming the retort or generator, a glass bottle-shaped vessel (B), which is the purifier, the two being connected together by a flexible tube. It will be seen that there are two tubes proceeding from the top of the purifier, one of which reaches nearly to the bottom of the vessel, and the other only just enters into the top. The former, or long tube, is the tube by which the purifier is attached to the retort; the latter, or short tube, is that which connects the purifier with the bag.
The method of charging the apparatus is as follows. Put into the retort about a pound and a half of oxygen mixture; pour into the purifier a sufficient quantity of water to about two-thirds fill it. Place the retort on a clear but not fierce fire, or over a gas furnace, and unite it to the _long_ tube of the purifier (B). Attach the _short_ pipe from the purifier to the gas-bag, and open all the taps, so that there may be a free passage-way from the retort to the bag. In a few minutes the gas will be given off, and will manifest itself by rising in bubbles through the water in the purifier. If a fierce fire be employed, the gas will be given off with undue rapidity and will pass through the purifier in volumes, so that it only becomes partially washed. A rapid but regular succession of gas is most to be desired. Should the gas come off with great violence, the retort should be lifted from the fire without disarranging the apparatus, and when the excessive rapidity is moderated it can be replaced. It is not, however, desirable to remove the retort from the fire if it can be avoided, for it not unfrequently happens that when the retort cools down, with only partially decomposed material in it, waste is the consequence, as it is not quite easy to re-establish the decomposing process. When the gas has entirely come off--which may be known by the cessation of the bubbles in the purifier--the top of the bag should be closed, the retort taken from the fire, detached from the purifier, and stood aside to cool. When cold, the residuum may be readily washed out with water, and when dry, the retort is ready for use again. The purifier should be emptied and washed out also. Always blow through the flexible tubes leading from the retort to the purifier, and wash out the metal tube of the retort in order to remove any solid material that may have been deposited there from the gas.