As laboratory experiments with gases require a certain preliminary knowledge, we will describe certain practical methods for the collection and preparation of gases. When in laboratory practice an intermittent supply of hydrogen (or other gas which is evolved without the aid of heat) is required the apparatus represented in fig. [21] is the most convenient. It consists of two bottles, having orifices at the bottom, in which corks with tubes are placed, and these tubes are connected by an india-rubber tube (sometimes furnished with a spring clamp). Zinc is placed in one bottle, and dilute sulphuric acid in the other. The neck of the former is closed by a cork, which is fitted with a gas-conducting tube with a stopcock. If the two bottles are connected with each other and the stopcock be opened, the acid will flow to the zinc and evolve hydrogen. If the stopcock be closed, the hydrogen will force out the acid from the bottle containing the zinc, and the action will cease. Or the vessel containing the acid may be placed at a lower level than that containing the zinc, when all the liquid will flow into it, and in order to start the action the acid vessel may be placed on a higher level than the other, and the acid will flow to the zinc. It can also be employed for collecting gases (as an aspirator or gasometer).
Fig. 22.—Continuous aspirator. The tube d should be more than 32 feet long.
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.
Fig. 23.—Gasholder.
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.
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.