290 A. W. Witkowski on the 



The determination of the compressibility of a gas at a set 

 of temperatures, together with its thermal expansion under a 

 single pressure (say the atmospheric), gives at once the ex- 

 pansion under any of the pressures employed. And vice versa, 

 if the compressibility at one chosen temperature be known, 

 it is sufficient to determine the thermal expansion of the gas 

 under different pressures, in order to obtain directly the 

 compressibility at any of the temperatures employed. 



The first of these two ways was followed by Wroblewski 

 in his above-mentioned work on hydrogen. In the present 

 investigation I have used experimental appliances modelled 

 on those of Wroblewski, and my theoretical aim was a 

 similar one; but I preferred to apply the second way of 

 experimenting, viz., the thermal expansion, instead of the 

 compressibility, has been considered as the principal subject 

 of the experiments. 



The following considerations have induced me to make 

 this change. First, the difficulty of measuring pressures 

 exactly. Since absolute manometers, suitable for laboratory 

 use and sufficiently trustworthy, are still to be invented, we 

 are compelled to use gas-manometers, founded on the com- 

 pressibility of gases (air or nitrogen). The pressure calculated 

 according to the indications of an instrument of this kind, 

 as well as the law of compressibility of the gas under investi- 

 gation, depend, in this case, directly on the assumed law of 

 compressibility of the gas used in the manometer. Suppose 

 we take this law as known, say through the experiments of 

 Amagat, then our results will be inextricably mingled with 

 these results. The method to be described presently, de- 

 pending on determinations of expansion under constant 

 pressure, on the other hand, furnishes values of expansion 

 entirely independent of any accepted law of compressibility : 

 a dependence of this kind remains only in the values of the 

 pressures applied. 



Another reason which induced me to depart from Wrob- 

 lewski's combination of the gas-manometer with the constant- 

 volume method of batterer, was the wish to invent a method 

 of constant sensibility as regards pressure- meaurements, 

 and the determinations of expansion and compressibility 

 as well. The gas-manometer is an instrument of variable 

 sensibility ; the higher the pressure measured, the less is the 

 exactness of measurement. On the other hand, the volu- 

 metric method is one of constant sensibility ; i. e. like incre- 

 ments of pressure yield approximately equal increments 

 of the quantity of gas. 



Instead of combining two methods of such opposite cha- 



