112 Prof. Richard Threlfall on the 



out. Using this number in the equation for -7 we find that 



n 



n'= (r009790) n, or the temperature-coefficient of the abso- 

 lute rigidity appears to be about +0*00012797 per degree 

 Centigrade. 



o J 



Determination of Coefficients of Expansion. 



The method of Matthiessen (Phil. Trans. 1866) was em- 

 ployed. I have in my possession a very fine set of copper 

 vessels fitted into one another, one of them being silvered out- 

 side and one inside. Consequently I can maintain five or six 

 litres of water at a high temperature with a very slow rate of 

 cooling. It was with this apparatus that the experiments 

 were made. As the method is well known it will perhaps 

 suffice to say that infinite pains were taken to get as good 

 results as the method will allow of. Everything that thought- 

 fulness could suggest was done, except the purchase of a 

 special balance, and yet the expansion of quartz appeared 

 irregular to a degree. In the final experiments the amount 

 of fused quartz available in the form of short thick rods was 

 more than 14 grammes — a quantity involving no inconsiderable 

 expenditure of time and oxygen in its preparation. In the 

 course of the work we plotted curves from various tables giving 

 the expansibility of water, and finally used the values given 

 by Matthiessen (loc. cit.). The small table in Everett's 

 ' Units and Physical Constants,' p. 39, is subject to a misprint 

 (discovered from the curves) in that it gives the density of 

 water at temperature 35° as 0'99469 instead of 0*99419. We 

 are inclined to believe that sticks of fused quartz are in 

 general in such a state of strain that the real expansion due 

 solely to rise of temperature is masked by other effects. This 

 suggests that the proposed use of fused quartz for thermometer- 

 bulbs may turn out to present a purely illusory advantage, 

 though on this point we would prefer to speak with all re- 

 servation. The accompanying table will give an idea of the 

 work done and the results arrived at. I ought to say that 

 many previous experiments had been made in my laboratory 

 by a student, Miss Robinson, and that my attention was 

 really drawn to the matter by the anomalous results she ob- 

 tained. The table gives the results of only the more reliable 

 experiments, i. e. those against which nothing is known ; any 

 want of sensitiveness of the balance will not account for the 

 abnormalities. 



