278 THE SIPHON-GAUGE 



In order to test the air-pump by means of the gauge, the air in 

 the receiver must be freed as much as possible from the aqueous 

 vapour which atmospheric air always contains in variable quan- 

 tities, and which, from causes which will be explained further on, 

 cannot be completely removed by pumping alone. Sulphuric acid 

 possesses the property of absorbing moisture ; a small tumbler or 

 jar is therefore filled with the acid to a height of about l cm and 

 placed by the side of the gauge under the receiver. The same acid 

 cannot be long used for the purpose of drying, but it must be 

 changed from time to time ; it should, however, not be thrown away, 

 but used for the preparation of hydrogen and in the galvanic 

 experiments to be described hereafter. 



A good air-pump should be capable of reducing the pressure in 

 the receiver to l mm and of maintaining this pressure if the pump is 

 allowed to stand for a day, the stop-cock being in the position III. 

 If these two conditions are not fulfilled, then either the stop-cock 

 or the plate are not air-tight. But if these two tests are satisfied, 

 the stop-cock is placed in position II. ; the mercury in the gauge 

 will then rise a little, because the air from the noxious space has 

 now entered the receiver ; the gauge should, however, in this posi- 

 tion remain at a constant height at least for several hours, even if 

 during this time the piston be slowly moved to and fro, without 

 altering the position of the stop-cock. If the gauge should show 

 a rise of pressure, either the connection between the cylinder and 

 the solid piece screwed to it is leaky, or the piston does not work 

 air-tight in the cylinder. An air-pump which is defective in this 

 respect when bought should be at once returned to the maker; 

 but if it should become leaky after having been used for some time, 

 it should be placed for repair in the hands of a skilful and con- 

 scientious workman. 



Since the column of mercury in a barometer falls, 

 when the pressure of the air upon the mercury in the 

 cistern is diminished by rarefaction, it follows that Hi 

 an open tube be dipped into mercury and the air in the 

 tube be rarefied, the mercury will rise in the tube to 

 a height which is nearly equal to the height of tin 

 barometer; it would evidently rise to the exact height 

 of the barometer, if by means of the pump the space 



