ELASTICITY OF AIB, 



45 



Fig. 3. 



The exhausting syringe used in practice differs in some particulars from 

 that which we have here described with a view to illustrate the principle of its 

 operation. The stop-cocks C and D, which would require constant manipula- 

 tion while the process of rarefaction is going forward, are dispensed with in 

 practice, and the elastic pressure of the air itself is mad* to act upon valves 

 which serve the purposes of these cocks. Let A, B, fig. 3, represent an ex- 

 hausting syringe, having a tube and stop-cock, C, proceeding from the lower 

 part, as already described. The tube C, is screwed to a very small aperture 

 in the bottom of the barrel. Across this aperture is stretched a small piece of 

 oiled silk, which is impervious to air. It is extended across the aperture so 

 loosely, that a slight pressure from below will produce an open space between 

 it and the surface of the bottom near the aperture capable of admitting air from 

 below, and yet so tight, that a pressure from above will cause it to lie close 

 against the bottom round the aperture, so as to stop the passage of air from 

 above. 



By this arrangement it is possible for air pressed with a sufficient force to 

 enter the barrel through the valve V. when the stop-cock C is opened ; but it 

 is impossible, on the other hand, for air pressing above the valve to escape 

 through it, since the pressure of the air only serves to render more close the 

 contact between the valve and the surface surrounding the aperture which it 

 covers. A small hole is pierced through the piston, extending from the lower 

 to the upper surface, and this hole at the upper surface is covered with an 

 oiled silk valve V, in the same manner as the aperture V, in the bottom. For 

 the reasons already assigned, it is, therefore, possible for air to pass up through 

 this hole in the piston, and escape at the upper surface ; but it is impossible 

 for air, by any pressure, to pass in the contrary direction, since such pressure 

 only renders the contact of the valve more intimate, and consequently causes 

 it to be more impervious to air. 



Let us suppose an instrument thus constructed to be attached to a vessel, 

 R, in which the rarefaction is to be produced, and the stop-cock C to be 

 opened. On raising the piston P, a vacuum will be produced between it and 

 the valve V. The piston-valve V' will now be pressed downward by the 



