240 



Prof. J. Dewar. Coefficients of the Cubical [Apr. 16, 



where W == true weight of substance examined, 

 w = true weight of platinum wire, 

 S = specific gravity of substance required, 

 D = specific gravity of liquid air, when B was observed, 

 d = specific gravity of gaseous air when A was observed, 

 s = specific gravity of platinum. 



From these relations we derive 



s - a 



instead of which we may write 

 A-C 



S = — — - D x correcting factor. 

 The correcting factor is 



B-C S - D 



^ d s-d 



D A-C 

 t)-d C~' 

 s-dA-B 



which in the present circumstances differs from unity only by a few 

 units on 10,000. 



In the subjoined Table I are given the values of A-C, A - B and 

 D for each observation. 



In the case of substances of less density than liquid air, a polished 

 copper ball weighing about 38 grammes was used as a sinker. The 

 details of the ice experiments, three in number, are given in Table II. 



Two experiments were made on compressed cylinders of solid car- 

 bonic acid. In the first of these the carbonic acid was compressed 

 dry, in the second, after a few drops of ether were added. The specific 

 gravities of solid mercury, iodine, and sulphur were also determined in 

 liquid air. The iodine was in the form of a compressed cylinder, but 

 the sulphur was a piece of a crystalline mass of native origin. 



The specific gravity of the actual portion of the substance weighed 

 in the liquid air was, with one or two exceptions, determined also at 

 the temperature of the laboratory, about 17° C. From the two sets of 

 observations the value of the mean coefficient of cubical expansion 

 between 17° C, and the temperature of liquid air, was calculated. 



In calculating coefficients of expansion, various forms may be given 

 to the formula employed, and correspondingly different results may be 

 obtained from the same set of observations. For short ranges of 

 temperature these results are practically identical, but this no longer 

 holds for a range of temperature such as we have in these experiments. 



