282 



EXPERIMENTS WITH THE AIR-PUMP 



to such an extent that the strength of the bladder 

 is less than this pressure, the bladder bursts with a 

 loud report. This will, however, only happen if the 

 exhaustion is rapid, as is the case when a large air- 

 pump is used; when the rarefaction is slow, the 

 bladder distends without bursting, and allows air to 

 pass through its pores. 



The pressure of the air is capable of forcing mercury 

 through the wooden bottom of a vessel, if the pressure 

 at the opposite side is removed. A small basin of 

 walnut- wood, as a in fig. 188, is fixed upon the 

 cylinder of a moderator lamp, the other end ol 

 the cylinder being ground flat, and placed upon the 



plate of the air - pump. 

 Within the wider portion 

 of the cylinder is placed 

 another little vessel, b, made 

 of some dense kind of wood. 

 When the air in the cylinder 

 is rarefied, the pressure of 

 the external air forces fine 

 globules between the fibres 

 forming the bottom of the 

 vessel a, and the drops fall 

 in the form of a ' rain ol 

 mercury ' into the vessel b. 



The rain must be observed ir 

 clo.se proximity ; even at a smal 

 distance the tiny globules are in 

 visible. The vessel 6, which pre 



FIG. 188 (i real size). vents the mercury from fallinj 



into the pump, should have : 

 polished surface inside. The small basin a should be turned on th< 

 lathe in such a manner that the fibres of the wood may be vertica 



