800 TENSION OF STEAM. 



The neck of the retort is slightly inclined downwards, 

 and the end of the tube allowed to dip below the sur- 

 face of water contained in a capacious basin or trough. 

 Heat is applied until bubbles no longer escape from 

 the tube, but the issuing steam is condensed with a 

 hissing noise by the cold water in the basin. The flame 

 is then removed, and as the temperature in the retort 

 falls, the pressure of the external air forces the water 

 up the tube, first slowly, but gradually more arid more 

 rapidly in consequence of the condensation of the steam 

 inside the retort by contact with the cold water which 

 enters. At last the water rushes in very fast and the 

 retort becomes almost completely filled. 



If the water in the basin and that in the retort have been boiled 

 previous to making the experiment, so as to drive out the dissolved 

 air, the retort will be filled completely. 



The peculiar hissing sound which is heard when 

 steam is condensed by being passed into water is caused 

 by the clashing together of particles of water which are 

 urged by atmospheric pressure to rush in from all sides 

 to occupy the space left vacuous by the condensed 

 steam. As water is very little compressible the sound 

 is very similar to that produced by striking solid bodies 

 together. 



When steam is generated in a vessel only partly 

 filled with water it will after a time expel the air 

 from the space above the water, and if the steam is 

 now condensed an almost perfect vacuum may be pro- 

 duced. If to the end of the tube which was immersed 

 in water in the previous experiment, but is now taken 

 out of it, an india-rubber tube be attached, and the 

 water in the retort be boiled for a few minutes, all the 



