Pneumatics. 73 



of the air, is this. Let a metallic cup be provided, 

 in whose bottom shall be fixed a cylinder of thorn, 

 or some other wood, about three inches long; 

 and let this cup and attached cylinder be placed 

 at the top of the receiver of the air-pump, so as 

 to exclude all external air. Then let quicksilver 

 be poured into this cup, and let a glass to re- 

 ceive it be placed within the receiver. Then, as 

 the rarefaction of the interior air proceeds, the 

 quicksilver will be forced, by the external pres- 

 sure, through the pores of the wood, and will be 

 seen to descend in a beautiful shower. 



Various facts in nature are explained by under- 

 standing the pressure and force of the air. The 

 word suction is founded on a vulgar error, for, in 

 fact, there is no such thing. In all cases where 

 suction is supposed, a vacuum or void is created, 

 and the pressure of the atmosphere forces the 

 fluid to fill up this void. Thus when children 

 suck at the breast, the mouth and lips of the 

 child act as an air-pump. The child swallows 

 the air in his mouth, while he holds the nipple 

 fast in his lips, so that none can come in that 

 way. A vacuum, of course, is created, and the 

 external air pressing on the breasts of the mother, 

 squeezes the milk into the infant's mouth. The 

 action of cupping glasses is explained on the 

 same principle. The air is driven out of the 

 cupping glass by means of heat, (as in the expe- 

 riment with the tea-cup,) that part of the body 

 where the glass is applied has therefore no pres- 



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