of Hydrogen and some other Gases. 315 



If the volume o£ the compressed gas is to be measured, 

 a screw-head similar to D with a short piece of glass tube is 

 screwed in the side hole ; this tube is then placed under a 

 receiving vessel for the gas. By turning the screw E very 

 slowly, the compressed gas can be let into the receiving- 

 vessel. 



§ 4. Procedure for Experiments. 

 (a) Adjustment of ilie Measuring Tube. 



The position of the measuring tube suspended from the 

 beam of the balance was accurately determined by means of 

 the microscope. The forward or backward displacement of 

 the tube from the central line joining the centres of the 

 pole-pieces was adjusted with an accuracy of O'l mm. by aid 

 of the lines marked on the pole-pieces. The height of the 

 tube could easily and quickly be adjusted by means of the 

 trifilar suspension system, with an accuracy of 0*05 mm. 

 The breadth of bifilar carrying the mirror was observed 

 with a Quincke microscope and adjusted with an accuracy 

 of 0*01 mm. The distance of the mirror from the scale and 

 telescope was 190 cm. When the breadth of the bifilar 

 suspension was 0'3 mm. the deflexion of one scale-division 

 corresponded to a vertical displacement of the measuring- 

 tube by about 1 X 10 " 3 mm. 



(b) Determination of the Mass. 



The mass of the gases subjected to the experiment was de- 

 termined by two different methods and the results compared 

 w 7 ith each other. The first method was to determine the mass 

 directly by weighing, that is, from the difference of weights, 

 first when the tube was filled with the compressed gas under 

 examination, and afterwards when it was evacuated. The 

 evacuation was always made after the tube was repeatedly 

 filled and evacuated several times with pure hydrogen by 

 means of a Gaede auxiliary pump, and then the weighing 

 w T as conducted with the utmost care. The second method 

 was to determine the mass by measuring the volume of the 

 gas collected in an eudiometer. Knowing the pressure and 

 temperature of the gas, we calculated the volume of the gas 

 at the standard pressure and temperature, and by multiplying 

 the density of the gas at the standard conditions by the 

 volume thus obtained, the mass of the gas was obtained. 

 These two methods were used in most of the experiments and 

 the results were found to agree very satisfactorily with each 

 other. But, for the reduction of the observed results, either 



