A CHEAP HOT STAGE C. C. KXPLINOER Lycoming College, Williamsport, Pennsylvania
AN ARTICLE appeared in a recent issue of THIS JOURNAL describing an electrically heated air-bath melting-point apparatus.' This is not the first time an air bath has been used effectively for melting-point determination^.^ It seems strange that most workers prefer the messy and sometimes even dangerous liquid baths that are almost universally indicated in laboratory directions. Many years of experience in our laboratories have clearly indicated the superiority of the air bath. The Walter heater unit was set up as indicated in his article with the following variations. Asbestos paper was used in its assembly with the heating element consisting of two feet of No. 28 Chrome1 wire. A Welch power unit supplied the current. This was mounted on a stand as in the original plan and placed about 2 cm. above the stage of a compound microscope. A piece of polaroid mounted between two glass slides was placed over the stage aperture, and three squares of asbestos paper with central holes laid over stage and polaroid served as heat insulation. I WALTER,J. H., "A useful melting-point apparatus," J. CHEM.EDUC.,30, 142 (1953). KIPI.INQER, C. C., AND A. YAUSSY,"Melting-point apparatus," Science, 93, 2407 (1941).
A one-inch objective was used and a piece of polaroid in the eyepiece served as an analyzer. A small strip of tinned iron about 4 om. long (cut from a discarded tin can), was bent so as to form a "U" trough which fitted the thermometer tube closely. A V-shaped notch was cut in one end of the trough and the prongs of the "V" were cut and bent so as to form a small platform. Small pieces of microscope cover glass were cut with a diamond point (a carborundum crystal serves nicely). The sample to be tested was placed between two of these pieces laid on the platform prongs and attached to the thermometer so that the plates touched the thermometer bulb. When properly prepared, the tin trough functions as does the safety clip on a pencil or pen and holds the plates firmly against the bulb. An asbestos paper shield with an observation hole cut in it and folded over the heater-tube eliminated reflections from the glass tubes. The open end of the heater-tube was closed with an asbestos shield. Capillary tubes can be used with this apparatus; however, the field definition is not good. The fact that the source of heat is external suggests that the specimen should be inclosed in glass of approximately the same thirkness as that of the thermometer bulb.
