_Gasholders_ are often used for collecting and holding gases. They are made of glass, copper, or tin plate. The usual form is shown in fig. 23. The lower vessel _B_ is made hermetically tight--_i.e._, impervious to gases--and is filled with water. A funnel is attached to this vessel (on several supports). The vessel _B_ communicates with the bottom of the funnel by a stopcock _b_ and a tube _a_, reaching to the bottom of the vessel _B_. If water be poured into the funnel and the stopcocks _a_ and _b_ opened, the water will run through _a_, and the air escape from the vessel _B_ by _b_. A glass tube _f_ runs up the side of the vessel _B_, with which it communicates at the top and bottom, and shows the amount of water and gas the gasholder contains. In order to fill the gasholder with a gas, it is first filled with water, the cocks _a_, _b_ and _e_ are closed, the nut _d_ unscrewed, and the end of the tube conducting the gas from the apparatus in which it is generated is passed into _d_. As the gas fills the gasholder, the water runs out at _d_. If the pressure of a gas be not greater than the atmospheric pressure and it be required to collect it in the gasholder, then the stopcock _e_ is put into communication with the space containing the gas. Then, having opened the orifice _d_, the gasholder acts like an aspirator; the gas will pass through _e_, and the water run out at _d_. If the cocks be closed, the gas collected in the gasholder may be easily preserved and transported. If it be desired to transfer this gas into another vessel, then a gas-conducting tube is attached to _e_, the cock _a_ opened, _b_ and _d_ closed, and the gas will then pass out at _e_, owing to its pressure in the apparatus being greater than the atmospheric pressure, due to the pressure of the water poured into the funnel. If it be required to fill a cylinder or flask with the gas, it is filled with water and inverted in the funnel, and the stopcocks _b_ and _a_ opened. Then water will run through _a_, and the gas will escape from the gasholder into the cylinder through _b_.