In laboratory practice, however, other forms of apparatus are generally employed for exhausting, collecting, and holding gases. We will here cite the most usual forms. An _aspirator_ usually consists of a vessel furnished with a stopcock at the bottom. A stout cork, through which a glass tube passes, is fixed into the neck of this vessel. If the vessel be filled up with water to the cork and the bottom stopcock is opened, then the water will run out and draw gas in. For this purpose the glass tube is connected with the apparatus from which it is desired to pump out or exhaust the gas. [Illustration: FIG. 22.--Continuous aspirator. The tube _d_ should be more than 32 feet long.] The aspirator represented in fig. 22 may be recommended for its continuous action. It consists of a tube _d_ which widens out at the top, the lower part being long and narrow. In the expanded upper portion _c_, two tubes are sealed; one, _e_, for drawing in the gas, whilst the other, _b_, is connected to the water supply _w_. The amount of water supplied through the tube _b_ must be less than the amount which can be carried off by the tube _d_. Owing to this the water in the tube _d_ will flow through it in cylinders alternating with cylinders of gas, which will be thus carried away. The gas which is drawn through may be collected from the end of the tube _d_, but this form of pump is usually employed where the air or gas aspirated is not to be collected. If the tube _d_ is of considerable length, say 40 ft. or more, a very fair vacuum will be produced, the amount of which is shown by the gauge _g_; it is often used for filtering under reduced pressure, as shown in the figure. If water be replaced by mercury, and the length of the tube _d_ be greater than 760 mm., the aspirator may be employed as an air-pump, and all the air may be exhausted from a limited space; for instance, by connecting _g_ with a hollow sphere. [Illustration: FIG. 23.--Gasholder.]