424 PROCEEDINGS OF THE AMERICAN ACADEMY. 



manent changes of volume of the glass hulb after subjecting to 100°. 

 That there is such an effect, very noticeable, is shown by the change of 

 zero of glass thermometers after subjecting to a high temperature. No 

 mention is made of the kind of glass ; if it were an American lead glass, 

 the effect would be comparatively large. The final value for the den- 

 sity of the solid as given by Mallet was 14.1932. This is the mean of 

 three values, 14.1S48, 14.1920, and 14.1929. 



The density of liquid mercury at the freezing temperature is given 

 in Landolt and BiJrnstein's tables as coming from Vicentini and Omo- 

 dei,*® but the value given by them is as a matter of fact quoted from a 

 paper of Ayrton and Perry. *^ Their method consisted in reading 

 simultaneously the same temperatures between 0° and —38°. 85 with a 

 mercury in glass thermometer and an air thermometer. The readings 

 of one plotted against the other are stated to be " very nearly linear," 

 from which the conclusion is drawn that the dilatation of mercury be- 

 tween 0° and —38°. 85 has the same constant value as that found by 

 Regnault to hold between 0° and +100". The density at — 38°.85 is 

 then calculated from the known density at 0°. No statement whatever 

 is made as to the degree of accuracy attained, or even as to the direc- 

 tion in which the plot of the air against the mercury thermometer de- 

 parts from linearity. However, the fact that one of the conclusions 

 drawn from the results is that mercury shows no maximum of density 

 in the neighborhood of the freezing point analogous to water, suggests 

 that the previous knowledge of this quantity was very imperfect in- 

 deed, and that the results of Ayrton and Perry themselves are not so 

 accurate as necessary for the present purpose. 



A direct determination of this change of volume with the same ap- 

 paratus as was used at higher pressures and temperatures would have 

 been possible, but inconvenient for several reasons. Chief of these was 

 the difficulty of running a thermostat of the requisite size with suffi- 

 cient constancy at this low temperature. Furthermore, if a result con- 

 cordant with the others could be found by an entirely different method, 

 the results by the two methods would mutually support each other, 

 making any consistent error in either improbable. 



The method used consisted in weighing a known quantity of mercury 

 under CS2 at different temperatures in the neighborhood of the melting 

 point. The weight varies with the temperature. When the tempera- 

 ture passes through the melting temperature, the mercury melts with 

 increase of volume, and the increased displacement causes a sudden 



*6 Vicentini and Omodei, Atti di Torino, 23, 38-43 (1887-1S8S); and 22, 

 28-47, 712-726. 



« Ayrton and Perry, Phil. Mag., 22, 325-327 (1886). 



