letters Nitrogen Controller
To the Editor: A liquid nitrogen level control somewhat simpler than that designed by Andrews and Current [J. CHEW E ~ u c . ,45, 803 (1968)l may be constructed from a 5-ml syringe and a small bore stopcock in the following manner. A short length of 4-mm Pyrex tubing is sealed at one end, wrapped with uninsulated copper wire, and attached via Tygon tubing to a vessel of about 25 ml volume. The small vessel is connected with another length of Tygon tubing to the syringe. The syringe, vessel, and tubing arrangement is filled to a slight positive pressure with oxygen gas. A 10-15 cm length of glass or metal rod is fastened with adhesive tape, glue, rubber bands, or whatever is convenient to the handle of a 1 mm bore stopcock. The other end of the rod is fastened with rubber hands to the plug of the syringe. The apparatus is clamped such that the stopcock is open when the syringe plug is extended, and closed when it is fully retracted into the barrel of the syringe. The stopcock controls the flow of low pressure nitrogen gas to t,he supply Dewar of liquid nitrogen. Finally, the sealed end of the Pyrex tube is placed in the Dewar being supplied at thc level which the liquid nitrogen is required to maintain. The upper part of the copper winding must extend above the lip of the Dewar. When the sealed Pyrex tube is partially submerged
in liquid nitrogen, the oxygen inside it condenses, the syringe plug retracts, and the flow of compressed nitrogen to the supply Dewar is shut off. As the liquid nitrogen level drops, enough heat is conducted to the pyrex tube by its copper wire to evaporate the oxygen. The syringe plug is extended, and the flow of compressed nitrogen is delivered to the liquid nitrogen supply Dewar. The above described apparatus may be constructed for approximately $7. THOMAS I,. NUNES
Paint Chemist
To the Editor: The insertion, "Chemistry Takes the Peel Out of Paint" [THIS JOUIINAL, 46, 173(1969)] appears to give me credit for the discovery of a way to reduce the peeling tendencies of paint. The work was done at the University of Southern Mssissippi under the direction of Professor Emeritus J . Scott Long whose contributions have covered a span of a t least three decades. My only role has been to call attention to the work which was done under the auspices of the Paint Research Institute, a fund-granting agency of which I am the resenrch director.
Volume 46, Number 6, June 1969
/
401
