1891.] on Liquids and Gases. 373 



raised througli one centimetre. If a pound be raised through two 

 feet, twice as much work is done as that of raising a pound through 

 one foot, and an amount equal to that of raising two pounds through 

 one foot. The measure of work is then the weight, multiplied by 

 the distance through which it is raised. When a gas expands against 

 pressure it does work. The gas may be supposed to be confined in 

 a vertical tube, and to propel a piston upwards against the pressure 

 of the atmosphere. If such a tube has a sectional area of one square 

 centimetre, the gas in expanding a centimetre up the tube lifts a 

 weight of nearly 1000 grams through one centimetre (for the pressure 

 of the atmosphere on a square centimetre of surface is nearly 1000 

 grams) ; that is, it does 1000 units of work, or ergs. So the work done 

 by a gas in expanding is measured by the change of volume multiplied 

 by the pressure. On the figure the change of volume is measured 

 horizontally, the change of pressure vertically. Hence the work done 

 is equivalent to the area on the diagram A B C D. 



If liquid as it exists at A change to gas as it exists at B, the 

 substance changes its volume, and may be made to do work. This 

 is familiar in the steam-engine, where work is done by water expand- 

 ing to steam, and so increasing its volume. The pressure does nor 

 alter during this change of volume if sufficient heat be supplied, hence 

 the work done during such a change is given by the rectangulat 

 area. 



Suppose that a man is conveying a trunk up to the first storey of a 

 house, he may do it in two (or perhaps a greater number of) ways. 

 He may put a ladder up to the drawing-room window, shoulder his 

 trunk, and deposit it directly on the first floor. Or he may go down 

 the area stairs, pass through the kitchen, up the kitchen stairs, up 

 the first flight, up the second flight, and down again to the first storey. 

 The end result is the same ; and he does the same amount of work in 

 both cases, so far as conveying the weight to a given height is con- 

 cerned ; because in going downstairs he has actually allowed work to 

 be done on him by the descent of the weight. 



Now the liquid in expanding to gas begins at a definite volume ; 

 it evaporates gradually to gas without altering pressure, heat being 

 of course communicated to it during the change, else it would cool 

 itself ; and it finally ends as gas. It increases its volume by a 

 definite amount at a definite pressure, and so does a definite amount 

 of work ; this work might be utilised in driving an engine. 



But if it pass continuously from liquid to gas, the starting-point 

 and the end point are both the same as before. An equal amount of 

 work has been done. But it has been done by going down the area- 

 stair, as it were, and over the round I described before. 



It is clear that a less amount of work has been done on the left- 

 hand side of the figure than was done before ; and a greater amount 

 on the right-hand side ; and if I have made my meaning clear, you 

 will see that as much less has been done on the one side, as more has 

 been done on the other ; that is, that the area of the figure B E H 



Vol. XIII. (No. 85,) 2 c 



