428 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The resistance points are to be taken as the correct ones, and the 

 mean of the three determinations, 0.002534, is accepted as the best 

 value. The departure from tlie previous value, 0.00260, is thus 

 nearly 3 per cent. 



This result may be used to give a more accurate value than that of 

 Mallet for the density of the solid mercury at the freezing point. As- 

 suming for the density of the liquid at the freezing point the value of 

 Ayrton and Perry, 13.690, the density of the solid is found to be 

 14.182 as against 14.193 of Mallet. 



This also enables a correction to be applied to the value of the 

 coefficient of dilatation of solid mercury as found by Dewar.^^ He 

 determined the density at —188°. 7 (in liquid air) and from Mallet's 

 value for the density at the freezing point calculated the coefficient of 

 thermal dilatation to be 0.0000887 over the range from — 38°. 8 to 

 —188°. 7. Substituting the above value for that of Mallet gives 

 0.0000907. Dewar's value for the density of mercury at the tempera- 

 ture of liquid air seems open to question, however. All of Dewar's 

 measurements of density at low temperatures depend on his value for 

 the density of liquid oxygen, which he gives as 1.137. The measure- 

 ments which he made on ice would seem to show a probable error 

 here. The value for the thermal dilatation of ice over the range 0° to 

 — 190° was found to be only half the known value from 0° to —20°. 

 So large a decrease seems hardly probable. A change in the funda- 

 mental constant so as to give a higher value to this dilatation would 

 also give a higher value to the thermal dilatation of mercury. Further- 

 more, a change in this direction would bring Dewar's value for the 

 dilatation of mercury into agreement with that of Grunmach,*^ namely, 

 0.000123 from — 38° to — 80°. This is apparently the only other value 

 we have. But Grunmach used a dilatometer method in which he made 

 no correction whatever for the effect of the glass envelope. The value 

 obtained by him for the change of volume on freezing with the same 

 apparatus is certainly wrong, namely, 5 per cent as against 3 per cent 

 found above, and this one fact is enough to cast discredit on the rest 

 of the work. There seems to be no value, therefore, which can be 

 accepted at present with confidence for the thermal dilatation of the 

 solid. 



In view of the apparent uncertainty as to the value of the thermal 

 dilatation of the solid, it may perhaps be allowed to force the above 

 weighings to give what information they can on this point. It is not 



" Dewar, Proc. Roy. Soc. Lon., 70, 237-246 (1902). 

 « Grunmach, Phys. ZS., 3, 133-136 (1902). 



