These are generated in gas bottles fitted, by grinding, with an S-formed tube; or in flasks to which a bent tube is adapted by means of a cork; in a common retort; or sometimes in iron bottles with a metal tube.* They are usually collected in vessels filled with water placed with their open end in a vessel of water. Pepy's Gas Solder is very useful for receiving, retaining, and transferring gases. The pneumatic trough consists of a vessel for holding water, with a shelf for sustaining the jars or bottles that are to be filled; these are filled by sinking them under water, and are then lifted on the shelf, above which the water rises, with their open end downwards. The beak of the retort, or bent tubes, are so placed that the gas issuing from them rises through the water into the vessel, and takes the place of the water in them. Some gases being very easily absorbed by water, are collected over mercury. Sometimes they are collected in dry bottles. For light gases, as ammonia, place a bottle in a vertical position with its mouth downwards over the tube from which the gas issues, which should touch the bottom of the bottle. When the bottle is filled with gas, and this escapes from the mouth, quietly withdraw the tube and close the bottle, still inverted, with a greased stopper. For gases heavier than air, as chlorine, or carbonic acid, the bottle must be plaeed with its mouth uppermost, and the tube delivering the gas must descend to the bottom of the bottle. When full of the gas, close it with the greased stopper. The tube connected with a flask in which a gas is generated should have a ball blown in it, into which asbestos may be introduced to arrest any particles thrown up by effervescence.

The following are the processes for procuring the principal gases: -