104 



ELEMENTS OF LABORATORY WORK 



Observations at higher pressures require inconveniently 

 long tubes ; but they have been found by special experiment 

 not to agree exactly with the results at low pressures. The 

 same results for low pressures, and the same slight divergence 

 from the general rule at high pressures, has been observed in 

 the case of most gases. We may say, for practical purposes, how- 

 ever, that the volumes of all gases vary inversely as the pressure. 



It is, of course, assumed that no mixture or union of the 

 bodies in contact (mercury and air) takes place ; and also that 

 the temperature is the same at each measurement of volume. 



(2, > VOLUME _ fr 



fig. 36. 



The various curves represent the relation between volume and pressure, for various 

 temperatures, of a given gas. The vertical distances of a given point, in any of these curves, 

 from a b and a c, represent the magnitude of the pressure and volume respectively. 



In order to measure corresponding changes we must take care 

 in every case that these changes only are being observed, or, at 

 any rate, we must take into account those modifications which 

 cannot be avoided, 



70, Graphic Representation of Correlative Changes by Dia- 

 gram. This mode of representing correlative changes may 

 be illustrated by the following example : Two straight lines 

 ab,ac are drawn at right angles to one another. The units of 

 length along a b are made to stand for units of volume, while 

 the units of length along a c stand for units of pressure ; that 



