Effusion of Argon, Helium, and some other Gases. 429 



riments were made with an initial pressure in the gas- reservoir 

 which was about 700 mm. in one series and about 500 mm. in 

 the others, the back-pressure rising from zero to 60 and 

 47*5 mm. respectively. The principal condition was, however, 

 that the apparatus should be such as to admit of measurements 

 being made with small quantities (20-30 c.c.) of the rarer 

 gases. It was necessary, therefore, to employ very small 

 apertures in order to reduce the unavoidable error in the 

 time-measurement * to a sufficiently small percentage. There 

 was a limit placed to the thickness of the partition, seeing that 

 it had to withstand pressures approaching that of an atmo- 

 sphere without collapsing, and so the conditions imposed 

 tended rather in the direction of viscosity-effects. As will be 

 seen in the sequel, such effects made themselves apparent. 

 They could doubtless have been avoided by the employment of 

 large quantities of gas and much larger apertures, but that was 

 not possible in the present case. The apparatus employed is 

 shown in the figure: a is the gas-reservoir, the gas being intro- 

 duced through the capillary tubing b c d by means of the gas- 

 syphon e. The pressure was read off in the closed barometer- 

 gauge/ by means of a mirror-glass scale. The effusion-plug, 

 or partition containing the effusion-aperture, was placed at or 

 near the lower extremity of the glass tube g ; h represents a 

 ground-glass joint with external mercury seal. During an 

 experiment, the taps i,j, k, and I being closed and m open, the 

 gas passed from the reservoir a into the vacuous receiver p 

 through the narrow aperture in the effusion-plug. The tube n 

 led to a Topler pump, which served to evacuate the apparatus 

 and to collect the gas again at the conclusion of an experiment. 

 Both gas-reservoir and receiver were kept at a constant tem- 

 perature by being immersed in a very large beaker of water. 

 The temperature of the water was maintained uniform and 

 constant by means of a stirrer with horizontal screw-shaped 

 blades driven by a Heinrici hot-air motor and an Ostwald 

 temperature-regulator filled with toluene. 



In order to make a measurement with the apparatus, the 

 mode of procedure was as follows : — The end q of the bent 

 capillary tube was drawn off to a fine point and sealed. The 

 taps i,j, /, and m were opened and the apparatus exhausted. 

 These taps were then closed, and the tube containing the gas 

 over mercury brought over the sealed point q by raising and 

 lowering the mercury trough. The drawn-out end, which 

 had been previously weakened by a file-scratch, was now 

 broken by pressing the top of the tube against it, and the 



* The time was measured by a stop-watch marking fifths of a second. 



