Fig. 41.
It is plain that when the level of the mercury in measuring a gas is read at l, the volume of the gas is greater than the volume of the mercury recorded, by twice the difference between the volume A of mercury measured, and that which would fill the tube to the level l, if its surface were plane.
The usual mode of finding the true volume of a gas collected over mercury is as follows:—
Place the graduated tube mouth upwards, introduce some mercury, and, after removing all bubbles, note the division at which it stands. Then add a few drops of solution of mercuric chloride; the surface of the mercury will become level, read and record its new position. Then, in any measurement, having observed that the mercury stands at n divisions of the tube, add twice the difference between the two positions of the mercury to n, and ascertain the volume which corresponds to this reading from the table of capacities.
To Calibrate the Tube of a Thermometer.—Detach a thread of mercury from half an inch to one inch in length from the body of the mercury. Move it from point to point throughout the length of the tube, and note its length in each position. If in one part it occupies a length of tube corresponding to eight degrees, and at another only seven degrees, then at the former point the value of each division is only seven-eighths of those at the latter position.
From the results obtained, a table of corrections for the thermometer should be prepared.
It is sometimes necessary to join soda glass to lead glass. In this case the edge of the lead glass tube may be bordered with white enamel before making the joint. Enough enamel must be used to prevent the lead and soda glasses from mingling at any point. The enamel is easily reduced, and must be heated in the oxidising flame. Dr. Ebert recommends Verre d’urane for this purpose. It is supplied by Herr Götze of Leipzig (Liebigstrasse).
[17] Originally suggested by Bunsen.