A MERCURY VACUUM PUMP 377
vide a vacuum sufficiently low for all common experiments inphysics and general science.The vacuum pump may be used as a substitute for the ex-
pensive pump purchased from supply companies, it may beused by individual pupils to supplement work with the moreefficient pump, or it may be used to demonstrate the principleof the mercury vacuum pump. The pump is especially usefulfor the latter purpose, as itsconstruction resembles the outlineof the pump as shown in many elementary physics and generalscience texts.
Suggestions: Ordinary laboratory glass tubing is used.If no large glass tubing is available for the aspirator, a test
tube or vial with the end filed off is a satisfactory substitute.The glass tubes in which photographic developing powders arepacked also may be used for this purpose.
If large glass tubing is available, it may be substituted for thestudent lamp chimney. Glass tubing used for this purposeshould have a diameter of not less than one inch.The neck of a distillation flask, filed off, is an excellent sub-
stitute for the side arm test tube used in the apparatus.A short section of rubber tubing placed over the arm of the
side arm test tube will prevent splattering of the mercury as itflows into the beaker.
Regular vacuum rubber tubing must be used. Ordinarylaboratory tubing will collapse as the vacuum is formed inside.The height of the apparatus may vary, provided the aspira-
tor is at least three feet from the floor.To operate the Pump: Connect the container to be exhausted
to EXHAUST by means of vacuum tubing. Close the clamp be-low the student lamp chimney and fill the chimney partly fullof mercury. Open the clamp slowly and allow the mercury toflow through the aspirator. When the mercury is near the bot-tom of the chimney close the clamp and pour the mercury fromthe beaker back into the chimney. Repeat the operation. Severaloperations are necessary to secure a high vacuum.
SALARIES... A study made by Professor Harold F. dark of Columbia reveals
that the public school teacher has drawn an average salary of (1,350 inthe years from 1920 to 1936. For doctors the average has been (4,850;lawyers, $4,730; and regular college teachers, $3,050. Draw your own con-clusions!The American Teacher.