THE COMPOSITION <>K WATKK. HYDROGEN 



127 



should he taken in the preparation of all gases. Time must be allowed 

 for the gas evolved to displace all the air from the apparatus, Other- 

 cork, through which u glass tube passes, is fixed into the neck of this vessel. If the 

 vessel I)*-' tilled u}) with wnter to the cork and the bottom stop-cock be 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. 



Tib aspirator represented in fig. 22 may be recommended for its continuous 

 action. It consists of a tube tl 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 //*. 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 rf, 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 con- 

 siderable 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 sup- 

 ports). The vessel B communicates with 

 the bottom of the funnel by a stop-cock 

 b and a tube a, reaching to the bottom of 

 the vessel B. If water be poured into the 

 funnel and the stop-cocks a and b opened, 

 the water will run through a, and the air 

 escape from the vessel B by b. A glass 

 tube / runs up the side of the vessel B, with 

 which it communicates at the top and bot- 

 tom, 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, ^=j 

 then the cock e is put into communication g 

 with the space containing the gas. Then, ^H 

 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 



Fig. 23. Gasholder. 



the cocks be closed, the gas collected in the gasholder may be easily preserved and trans- 

 ported. 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 then the gas will pass out 

 at e, owing to its pressure in the apparatus being greater than the atmospheric pressure 



