EXPANSIVE FORCE SHOWN BY EXPERIMENT. 19 



the tube, and the space in the flask has become 

 quite clear, indicating that it is now filled by pure 

 steam and not by particles of water mixed with 

 air. The pinch-cock is then closed and the flask imme- 

 diately removed from the lamp, or the pressure of the 

 steam inside might burst it. The air in the flask has 

 been displaced by steam, and after cooling, which may 

 be hastened by placing the flask in cold water, the steam 

 is reconverted into water, leaving in the flask a space 

 which, though not a perfect vacuum, is sufficiently near 

 it for the present experiment. The second flask is now 

 filled with smoke ,by inserting through its neck, to 

 about the middle of the flask, a wire to which a piece of 

 ignited fusee is attached. As soon as the bottle is 



o 



filled with dense smoke, the wire and fusee is withdrawn 

 and the flask set upon its cork. On opening the pinch- 

 cock the mixture of air and smoke immediately enters 

 the vacuous flask, and as the air now fills a larger 

 volume, it is more rarefied than previously ; this is 

 shown by the clearer appearance of the smoke. If the 

 flask which contains the smoke is not connected with 

 the vacuous space, but simply left to stand open, the 

 smoke would also have expanded, though much more 

 slowly, because the surrounding air retards the expan- 

 sion. 



It can be ascertained whether the apparatus is air-tight by 

 immersing the flask in water, so that the cork is covered, when on 

 blowing strongly through the tube small air-bubbles will rise from 

 every leak. Should the projecting piece of glass tube be too short 

 for this operation, a longer india-rubber tube may be temporarily 

 attached. The tubing generally used for connecting the various 

 portions of apparatus consists mostly of vulcanised india-rubber, 

 or caoutchouc which has been made more soft and elastic by an 

 admixture of sulphur. Such tubing is of a grey colour; another 



c 2 



