. 



EXPERIMENTS WITH THE AIR-PUMP 281 



The pressure of the atmosphere may be manifested without 

 Magdeburg hemispheres by means of a common receiver of glass. 

 After a few strokes of the piston, "force will be required to remove 

 ,he receiver from the plate. In this case also care should be taken 

 lot actually to lift the receiver from, the plate, otherwise both 

 Dump and receiver might be seriously injured. The receiver 

 should never be taken off, unless the air has been previously let in 

 igain. Even in that case there remains adhesion enough, espe- 

 cially when the flat edge of the receiver is somewhat wide, to 

 squire some exertion for the separation ; it should be turned round 

 Between both hands until it moves easily upon the plate, and then 

 lifted off. 



The pressure of the atmosphere may be rendered sensible without 

 ,;he air-pump in the following manner. Grind the edge of a glass 

 unnel, 4 or 5 cm wide, upon a glass plate with emery powder, in the 

 iame manner as was done previously with the edges of the vessels 

 x>r the hydrostatic experiments (page 167); push a short piece of 

 ndia-rubber tubing over the tube of the funnel and provide it with 

 i pinch-cock. Lard the flat edge of the funnel, place it upon an 

 idhesion-plate, open the pinch-cock, suck strongly at the end of 

 he india-rubber tube, and close the pinch-cock before you have 

 ;eased sucking. The pressure inside the funnel is now so much 

 liminished, that a force of from 3 to I0 k & r would be required to 

 separate the plate from the funnel. The application of such a force 

 vould evidently endanger the plate and the funnel, both being of 

 ^lass ; but by placing the fingers of the left hand round the edge of 

 lie plate and seizing the tube of the funnel with the right hand, a 

 :onsiderable pull may be exerted with safety. 



strong hollow cylinder of glass or metal, open at 

 h ends, may be used to show the crushing force of 

 he pressure of the atmosphere. One edge is ground 

 lat, and upon the other end is tied a bladder, pre- 

 iously softened in lukewarm water, then tightly 

 itretched, and fastened by several turns of string, 

 l^he flat edge is now larded and placed upon the plate 

 f the pump. When the air inside the cylinder is 

 ;xhausted, the bladder is bent in by the pressure of the 

 xternal air, and when the rarefaction can be carried 



