Simple Aid for Counting Crowded Plates.

'Thus it is necessary to turn the core through a ninety degree arc only. This has been found to be of dec-ded advantage when the stop-cock is operated mechanically. A similar stop-cock with a bronze core has been made. The advantage of the steel core is the fact that it can be used in positions in which there is a possibility of its coming in contact with mercury. These stop-cocks are used in gas analysis apparatus. The device shown at B in Fig. 1 consists of four, two-way stop-cocks, in one unit, each sealed by mercury under a pressure of about three centimeters. Over a period of about a year it has given satisfactory service as part of a device for obtaining aliquot samples of air. The core and shell of the mercury-sealed stop-cock were machined until a very close fit was obtained and were then run together in the lathe, well lubricated with a light grade of oil, until the parts were worn enough to permit reasonably free movement. Taper of the core is 0.003 mm per cm length. When sealed with graphite, no leaks could be found in the mercury-sealed stop-cock under pressure of 30 em of water. C. F. WINCHESTER COLLEGE OF AGRICULTURE UNIVERSITY OF CALIFORNIA DAVIS