434 PROCEEDINGS OF THE AjSIERICAN ACADEMY. 



determine the change in the temperature of the liquid with certainty 

 to 0.01° C, but the absolute temperature of the liquid was not known 

 closer than 0.1° C. 



Compressibility of Glass. 



Since the liquid under examination was enclosed in a glass bulb 

 which changed its capacity with change of pressure on the liquid, the 

 change of volume in the liquid was partly hidden. As the piezometer 

 contained a platinum thermometer, the compressibility of the glass of 

 which it was made could not be determined directly in the usual way, by 

 use of mercury. Accordingly another piezometer was made which dif- 

 fered from the first only in this, — that it contained no platinum ther- 

 mometer. Its bulb was made from the same piece of glass tube from 

 which the bulb of the other piezometer was taken. Its capillary stem 

 had a mean cross section of 0.00541 (cm.)'' and had been calibrated in the 

 usual way. The capacity of this piezometer, determined by weighing 

 it empty and then weighing it filled with mercury, was found to be 

 63.21 (cm.)". The mercury being at 20° C, observations were made in 

 the usual way on the apparent change of volume of the mercury in 

 glass for a change of pressure, external and internal, of three atmos- 

 pheres. Before the measurement of the change of volume a wait of 

 ten minutes was allowed for the heat of compression to disappear. 

 Experiment showed, ki — k-i = l.ll X 10~®, where 



ki — compressibility of mercury at 20° C. 

 ^2 = " " glass " " " 



This value is the mean of twenty observations, — ten made by in- 

 creasing and ten by decreasing the pressure. According to Amagat^ 

 ki — 3.92 X 10-^ whence k^ = 2.15 X 10"^ This value is in good 

 agreement with the value given by Amagat * and with that given by 

 Richards and Stull.^ 



Expansion of Glass. 



Not only does the capacity of the piezometer change when the pres- 

 sure on it is changed, but also the capacity of the dilatometer changes 

 when its temperature is increased or decreased. One must therefore 

 know the coefficient of expansion of glass in order to make the neces- 

 sary correction in the work on expansion of liquids. By the ordinary 



8 Comp. Rend., 108, 228 (1880). * Loo. cit. 



^ Publication No. 7 of Carnegie Institution of Washington. 



