ON EUDIOMETERS. 287 



the height is read by means of a cathetometer, which 

 consists of a telescope sliding on a vertical rod which has 

 cross wires in the eyepiece, and enables you to get the 

 exact surface of the mercury. Having got the instrument 

 'into its proper position, you read the divisions and estimate 

 the lOths at which the mercury stands. This process is 

 repeated over and over again until the tube is full. If the 

 tube were truly cylindrical each volume of mercury which 

 is introduced would raise the column exactly to the same 

 extent ; but as a tube never is perfectly cylindrical, a 

 calculation has to be made to determine the value of each 

 separate division. When mercury is contained in a glass 

 vessel the surface is not plane in consequence of capillarity 

 the surface is in fact convex. Now, during the calibration 

 of the eudiometer the convexity of the surface is upwards, 

 that is, towards the open end of the tube, whereas when 

 the eudiometer is employed for measuring the volume 

 of a gas, the curvature of the mercury is in the opposite 

 direction. The volume of mercury which is measured by 

 the calibrating tube is therefore a little less than the 

 volume of gas that would be contained in the tube when 

 the highest part of the surface of the mercury is at the 

 same division on the eudiometer. It is necessary to deter- 

 mine this volume, which is done by noting the height at 

 which the mercury stands when the eudiometer is inverted, 

 and then introducing a small quantity of solution of 

 mercuric chloride (corrosive sublimate), which attacks the 

 mercury, altering the surface tension and making the 

 surface horizontal. The distance which the mercury sinks 

 in the centre of the tube is noted, and twice the volume 

 indicated by the falling of the mercury is the volume 

 which would escape measurement if this precaution were 

 neglected. In all measurements made with the eudiometer 

 this error of meniscus has to be added to the observed 

 volume, or what comes to the same thing ; twice the dis- 

 tance that the mercury has fallen is subtracted from the 

 observed height of the column of mercury. The eudiometer 

 is now ready for use. 



It is of course extremely necessary in all measurements 

 of gas analysis to be very careful to avoid parallax in the 

 reading. If the eye is a little too high or too low you 

 will read the instrumt nt too h ; gh or too low. Thus I can, 



