COLLIGATIVE RELATIONS AND SCIENTIFIC LAWS 119 



is both invention and discovery. Purely as a concept it is an inven- 

 tion; but its scientific value becomes established only with the dis- 

 covery in our experience of certain items readily interpretable in 

 terms of this concept. 



Boyle s law. In this new context let us examine in greater detail the 

 complex passage, from observational data to scientific law, that was 

 only sketched when the law of the lever was discussed in Chapter II. 

 Imagine that we invent sl J-tube apparatus, like Boyle's own, with a 

 pocket of air trapped by mercury in the closed short limb. We observe 



\ / 



'B 



y— -L 



At left: A small volume of air, trapped in the closed arm of the J-tube, 

 is compressed into a smaller volume when more mercury is added. 

 The total pressure acting on the confined gas is m.ade up of the hydro- 

 static pressure, p.i, plus the atmospheric pressure, pa, acting on the 

 mercury surface in the open arm of the J-tube. 



At right: A barometer permits the determination of the atmospheric 

 pressure which, acting on the surface of the mercury in the dish, holds 

 the mercury in the evacuated tube at a height that reflects the magni- 

 tude uf the atmospheric pressure, Pr. 



