CONNECTED WITH FLUIDS. 



A* long as the mrefection of the air is not very great the quantity of 

 air which ia included between successive portions of mercury is sufficient to act 

 M a sort of buffer, but as the rarefaction increases the portions of mercury 

 ooinQ together more abruptly, and produce a sound which becomes sharper as 

 the vacuum becomes more perfect. 



After the mercury-pump has been in action for some hours the quantity 

 of matter remaining in the vessel is very small. If the tubes have been joined 

 by means of caoutchouc connections there is a trace of gaseous matter emitted 

 by the caoutchouc. It is therefore necessary, when a very perfect vacuum is 

 desired, to moke all the joints " hermetical " by fusing the glass. Still, however, 

 there remains a trace of matter. The vapour of mercury is, of course, present, 

 and the sides of the glass vessel retain water very strongly, and part with 

 it very slowly, when all other matter is removed. 



By passing a little strong sulphuric acid through the pump along with the 

 mercury, vapours both of mercury and of water may be in great measure 

 removed. 



MM. Kundt and Warburg have got rid of an additional quantity of water- 

 substance by heating the vessel to as high a temperature as the glass will 

 bear while the pump was kept in action. 



A method which has been long in use for getting a good vacuum is to 

 place in the vessel a stick of fused potash, and to fill it with carbonic acid, 

 and, after exhausting as much as possible, to seal up the vessel. The potash 

 is then heated, and when it has again become cold, most of the remaining 

 carbonic acid has combined with the potash. 



Another method, employed by Professor Dewar, is to place in a compart- 

 ment of the vessel a piece of freshly heated cocoa-nut charcoal, and to heat 

 it strongly during the last stages of the exhaustion by the mercury-pump. 

 The vessel is then sealed up, and as the charcoal cools it absorbs a very large 

 proportion of the gases remaining in the vessel. 



The interior of the vessel, after exhaustion, is found to be possessed of 

 very remarkable properties. 



One of these properties furnishes a convenient test of the completeness of 

 the exhaustion. The vessel is provided with two metallic electrodes, the ends 

 of which within the vessel are within a quarter of an inch of each other. When 

 the vessel contains air at the ordinary pressure a considerable electromotive 

 force is required to produce an electric discharge across this interval. As the 



