EVAPORATION OF WATER. 



The thermometer would rise, the piston maintaining its position, 

 and this would continue until the thermometer would rise to the 

 temperature of 212. Upon rising to that temperature the 

 thermometer would remain stationary, and at the same time the 

 piston, P, would begin to rise, leaving a space apparently empty 

 between it and the surface of the water. The lamp, or fire, still 

 continuing to impart the same heat to the water, the thermometer 

 nevertheless will remain stationary at 212, but the piston will 

 continue to rise higher and higher in the tube, and if the depth of 

 the water in the bottom of the tube be measured, it will be found 

 that it is constantly diminished. If a sufficiently exact measure- 

 ment of the decrease of the depth of water, and the height to 

 which the piston is raised could be made, it would be found that 

 the one would bear a fixed and invariable proportion to the other, 

 the height of the piston being always 1669 times the decrease ot % 

 the depth of water. . -. 



In fine, if this process were continued for a sufficient time, and 

 if the tube had sufficient length, the water would altogether 

 disappear from the bottom of the tube, and the piston would be 

 raised 1669 inches, or 139 feet very nearly. For the convenience 

 of round numbers, in a case where the most extreme arithmetical 

 accuracy is not needed, we shall then assume that the piston 

 loaded with 151b. has been raised 140 feet. 



8. After this has taken place the tube below the piston will 

 appear to be quite empty, the water having disappeared, and no 

 visible matter having taken its place. If, however, the tube and 

 its contents were weighed, they would be found to have the same 

 weight precisely as they had when the water was deposited under 

 the piston. 



The phenomenon is easily explained. The heat applied to the 

 tube has converted the visible liquid water into invisible steam. 

 It is a great but very common error to suppose that the whitish 

 cloudy vapour which is seen to issue from the safety valve of an 

 engine, or the funnel of a locomotive, or the spout of a boiling 

 kettle, is steam. The semi-transparent matter which floats in the 

 air, and continues to be visible for some time after it escapes from 

 the boiler, is in fact not steam, but water existing in very minute 

 particles, produced by the condensation of the steam by the 

 contact of the colder air. When those particles coalesce and form 

 small drops of water, they either fall to the ground or are 

 evaporated at a lower temperature, and in either case disappear. 

 If the vapour issuing from the safety valve of an engine, or the 

 spout of a boiling kettle, be closely examined, it will not be found 

 to have that cloudy semi-transparent appearance until it has 

 passed to some distance from the point from which it issues. 



199 



