2-4 PROCEEDINGS OP THE AMERICAN ACADEMY 



been completely filled with mercury by suction, the weight W, corre- 

 sponding to their total capacity was obtained in a similar way. The 

 required temperature could now be calculated by means of the well- 

 known law of Charles : — 



T°' + 273°.2 = (7-°. + 273^2) ^^. -^^ [1 J^ {T^-T°') /t] 



It will be noted that as the mercury columns, including the heights 

 of the barometer, were all measured at the same constant tempera- 

 ture ; and, as we are dealing with relative values only, no reductions 

 are necessary. Moreover, an error of one tenth of a millimetre in the 



value of jj, Y would make, in determining the boiling point of sul- 

 phur (448°), a difference of only one eighth of a degree, so that meas- 

 urements of these heights are sufficiently close, if accurate, to one half 

 a millimetre, and might even be made with a common rule. The 

 most uncertain element in the formula is the expansion of glass ; but 

 if the bulbs are made of flint glass (lead glass) tubing, such as is used 

 in this neighborhood for ornamental ware, the mean coefficient of ex- 

 pansion will vary very little from 0.000025, if the temperature does 

 not exceed that at which the glass begins to soften. The rate of ex- 

 pansion of flint glass is not only less than that of crown, but it is also 

 more constant, and increases very slowly with the temperature. Flint 

 glass is therefore better adapted for the use we are describing. The 

 expansion of the glass used in our experiments was carefully deter- 

 mined, and found to have the value given above, within two or three 

 tenths of a unit in the last place. A difference of one unit in this 

 place would make a difference of one third of a degree in the boiling 

 point of sulphur. 



In order to test the accuracy of this method, Mr. Bennett made 

 four determinations of the boiling point of sulphur under different 

 barometric conditions, which in the following table are compared with 

 the results of Regnault, reduced to the corresponding pressures : — 



Boiling Point of Sulphur. 



776.7 448.2 448.7 —0.5 



