410 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The Tliermal Dilatation of Kerosene at Constant Pressure. 



Since the accuracy desired was not very high, the maximum value 

 of the correction being 2 per cent, a procedure was adopted which was 

 rapid, but not accurate enough to employ in determining this quantity 

 as a physical constant interesting in itself. The method consisted 

 essentially in filling the entire apparatus with kerosene, lower as well 

 as upper cylinder, immersing the lower cylinder in the constant tem- 

 perature bath, and plotting piston displacement against pressure. 

 Then the lower cylinder was allowed to come to room temperature, and 

 again the piston displacement and the corresponding pressures noted. 

 The difference of displacement at the two temperatures gives the 

 change of volume of the kerosene which was in the lower cylinder on 

 expanding from the one temperature to the other. It is to be noticed 

 that this gives the thermal dilatation of a quantity of kerosene of un- 

 known mass but of known volume. To find the dilatation as ordinarily 

 defined, we should require the compressibility, so that we might know 

 the original volume of the kerosene which at the given pressure com- 

 pletely filled the lower cylinder. But the data as given by the experi- 

 ment are exactly the data needed in making the correction. For 

 suppose the change of volume of the mercury on melting at a certain 

 temperature and pressure is approximately 1 cm.^. We require to 

 know how much this 1 cm." of displaced kerosene has changed in 

 volume on passing from one temperature to another at constant pres- 

 sure, and we are not interested at all in the original volume of this 



1 cm.^ of kerosene. 



The procedure as roughly outlined above is evidently liable to a 

 multitude of errors. It is exactly similar to the corresponding method 

 of obtaining compressibilities against which objections were made in 

 the introduction. But it is to be said in extenuation that the cor- 

 rection is small, and that the maximum correction comes at the low 

 pressure where the objections of the introduction have little weight. 

 The correction at — 20° of 2 per cent corresponds to only 3500 kgm. 

 At this pressure and with a cylinder seasoned by exposing to 24,000 kgm. 

 there need be no hesitation even in applying the method to accurate 

 determinations of the compressibility. The correction at 0° and 

 7500 kgm. has already dropped to 0.6 per cent. 



The corrections to apply in determining this maximum correction of 



2 per cent are the corrections on the volume of the lower cylinder 

 brought about by the changes in temperature and pressure. The 

 volume was found by weighing the lower cylinder when full of mercury. 

 To be subtracted from this volume was the volume of a copper block 



