LIQUIDS AND GASES. 311 



iii^i" ;i centimeter up the tube lifts a weight of iieaily 1 ,000 grants through 

 1 centimeter, for the preissure of tin? atmostjjhere on a S((uare (;enti- 

 meter of surface is nearly 1,000 grains — that is, it (h)es 1,000 units of 

 work, or ergs. So the work done by a gas in expanding is measured 

 by the change of volume multi]»licd by the ])ressure. On tlie figuic, tiie 

 change of volume is measmcd horizontally, the change of pressure ver- 

 tically. Hence the work done is equivalent to tlie area ABC 7> on 

 the figure. 



If li(piid as it exists at A change to gas as it exists at B, the sub- 

 stance changes its volume aud may be made to do work. This is 

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

 to steam aud so increasing its v<dume. The pressure does not alter 

 during this change of volume if suflflcieut heat be supplied; hence the 

 work done during such a change is given by the rectangular area. 



Suppose that a man is conveying a trunk u]} to the first story of a 

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

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

 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 story. The end 

 result is the same; aud he does the same amount of work in both cases 

 so far as conveying the weight to a- given height is concerned, because 

 in going down stairs he has actually allowed work to be done on him 

 by the descent of the weight. 



Now, the li(iuid in expamling to gas begins at a definite volume; it 

 evaporates gradually to gas without altering lux'ssure, heat being, of 

 c<mrse, communi<;ated to it (hiring the change, else it would cool itself; 

 and it finally ends as gas. It increases its volume by a definite amount 

 at a detinite pi-essure, and so does a definite amount of work. This 

 work might be utilized in driving an engine. 



r>ut if it i)ass continuously from licpiid to gas, the starting ])oint and 

 the end i)oint are both the same as before. An e(|ual anu>unt 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 tliat a less auu)unt of work has been done ontiie h^ft hand 

 side of the figure than was done belbre, and a greater amount on the 

 right-hand side; and if I have made my mcainng clear you will s<'e 

 that as nuich 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 /f must be 

 equal to that of the figure A F H. Dr. Young and 1 have tried this 

 experimentally — that is, by measuring the calculated areas — an<l we 

 found them to be equal. 



This can Ik^ shown to you easily by a simple device, namely, taking 

 them out and weighing them. As this diagram is an exact represen- 

 tation of the results of our experiments with ethci- th(^ device^ can be 

 put in luactice. We can detach these areas, which are cut out in tin, 



