342 Mr. Take Sone on the Magnetic Susceptibilities 



after an hour the observation is repeated. These processes 

 are repeated so long as there is no relative change in the 

 readings in these microscopes. The length of the eudio- 

 meter is about 50 cm. and its inner diameter 17 mm. ; while 

 the inner diameter of the tube b is 4 mm. 



Let us now consider the change of the 

 height of mercury in the tube 6, when 

 a change of volume of the gas in the 

 eudiometer takes place by electric dis- 

 charge. 



Let S and 5 be the sectional areas of 

 tubes a and b, P and p be the initial and 

 final pressures of the gas in the eudio- 

 meter respectively (fig. 10) ; then if the M: 

 meniscus of the mercury in tube a is 

 raised by SH, w r hile that in tube b is 

 lowered by hh, we have the 

 relations : — 



following 





Kg. 



10. 





•x-^ 





a. 





b 





S 



B . 



T\ — 







X 





SSH = *67i, and P-p = $K + hh 

 Hence we have 



P-p: 



or 





If the quantity of oxygen previously contained in the 

 hydrogen be very small compared with the total quantity of 

 the gas and the hydrogen was perfectly dry before discharge, 

 the water formed by discharge will remain as vapour, and 

 since one volume of oxygen and two volumes of hydrogen 

 form two volumes of water vapour, the total volume change 

 of the gas due to the discharge is just equal to the volume 

 formerly occupied by the oxygen gas. Hence the term in 

 the left side of the above equation is the ratio of the partial 

 pressure due to the oxygen to the total pressure of the gas. 

 Suppose S7i to be 1/66 mm., which is the smallest division 

 of the scale of the microscope, then we have, from the above 

 equation, 



P-p_ 1-055 1 _,_ 1 



P ~ 66 x 760 ~ 47500 ~ 50000 * 



Hence we see that if the height of the mercury meniscus 

 in tube b does not change by more than 1/3 division of the 



