ELASTICITY OF AIR. 



an open mouth is connected with an exhausting syringe. The mouth is ap- 

 plied in air-tight contact with the skin, and by working the syringe, a part of 

 the air is withdrawn from the vessel, and consequently the skin within the 

 mouth of the vessel is relieved from its pressure. All the other parts of the 

 body, however, being still subject to the atmospheric pressure, and the elastic 

 force of the fluids contained in the body having an equal degree of tension, 

 that part of the skin which is thus relieved from the pressure will be swelled 

 out, and will have the appearance of being sucked into the cupping-glass. If 

 the skin be punctured by lancets, the blood will thus be drawn from it in a 

 peculiar manner. 



That the presence of air is necessary for the transmission of sound, may be 

 strikingly illustrated by the air-pump. A small apparatus, fig. 11, which, by 



Fig. 11. 



being drawn upward and downward alternately, causes a bell to ring, is placed 

 on the pump-plate, and covered by a receiver with an open top. A brass cover, 

 furnished with a sliding rod, is placed upon this. The sliding rod is termina- 

 ted in a hook, which catches the apparatus, and by which it may be alternately 

 raised and lowered, without allowing any air to pass into the receiver. The 

 apparatus being thus suspended in the receiver by a silken thread, so that it 

 shall not touch the bottom or sides, let the air be exhausted by the pump. 

 When the rarefaction has been carried to a sufficient extent, let the rod be al- 

 ternately raised and lowered, so that the bell shall ring. It will be found to be 

 inaudible. 



If the air be now gradually admitted, the sound will at first be barely audible, 

 but will become louder by degrees, until the receiver is again filled with air, in 

 the same state as the external atmosphere. In this experiment care must be 

 taken not to let the sounding apparatus rest on the pump-plate, for it will then 

 communicate a vibration to that, which will finally affect the external air and 

 produce a sound. 



THE CONDENSING SYRINGE. 



The condensing syringe is an instrument by which a greater quantity of air 

 may be forced into a vessel than that vessel contains when it has a free com- 

 munication with the external atmosphere. 



