correction applied as found in Table I, to calibrate all additional melting point apparatus thermometers. A thermometer with a range of 0" to 350' C. in 1" divisions calibrated every 50" of the range was then used in the actual apparatus, Constructional Details. Figure 1 shows the assembled melting point apparatus with all parts in place.
The Kovar metal to glass seal is made with the following Corning glasses: S o . 774 (Pyrex) to 3320 (Uranium) to 772 (Nonex) to 7052 (Kovar sealing glass). The Chrome1 wire is silver-soldered to a 0.40-inch copper lead, and the copper lead wire is soldered into the Kovar metal tube, also with silver solder. The leads connecting the appa-
ratus and the variable transformer are made from a braided 0.40-inch copper wire covered with plastic insulation tubing to give flexibility, and a suitable electric plug is attached to fit the variable transformer. A is the melting point capillary tube holder with four small guides to support the capillary tubes. The 19-mm. disk a t the bottom was cut from a section of borosilicate glass rod 19 mm. in diameter. The two air holes in the tube holder are important, as they relieve the pressure in the apparatus as the silicon oil expands. ' B is the adapter for the spiral stirrer; this serves as a guide for the stirrer. Enough clearance should be present between the adapter and stirrer shaft to permit a pressure release as the silicone oil is heated.
C is the stirrer with the spiral blade, The blade is made from a section of I/ginch thick borosilicate plate glass cut 20 mm. in width. When the spiral is fabricated it will narrow about 2 mni. to a finished product of 18 mm. in width. It is important that the spiral be fabricated so as to circulate the silicone oil in a counterclockwise direction, so that the silicone oil is pulled through the agitator and down through the test chamber. D is the test thermometer. Figure 2, a voltage input chart, shows voltage required in relation to a temperature rise per minute. A patent on this apparatus has been applied for; the apparatus is manufactured by Kontes Glass Co., Vineland, ? J i, 1
Device for Orienting Small Crystals under the Microscope Betty J. Steinbach' and Thomas R.
P. Gibb,
optical characterization of small I crystals is greatly expedited by device which permits a single crystal or THE
a
fragment to be oriented in various nays under the polarizing microscope, particularly when the crystal has a preferred habit or cleavage-for example, boric acid-or only a single small crystal is available. The novice finds it much easier to visualize the optic directions and to measure optical properties with the aid of such a device, as the crystal may easily be turned to show centered or a t least recognizable interference figures. The axial rotation stage (available from Kenneth A. Dawson Co., Belmont, Mass.) was developed in the hope of satisfying the need for a crystal-orienting device intermediate between makeshifts of limited applicability (9) and the intricate universal stage of Federov ( I ) . The device described permits orientation on three axes of rotation, allowing continuous examination of interference figures and usually measurement of all indices of refraction. Rotation about one of these axes is limited to 20" or 30°, depending on the objective used. Nevertheless, this limited rotation is a desirable feature, as it frequently eliminates the need for precise alignment of the crystal with the shaft. It is particularly desirable as a means of obtaining centered figures. In the construction of the stage (Figure 1) the following specifications are essential. The center of rotation of the yoke, F , must be slightly more than the thickness of a microscope slide above the top of the supporting plate A , as the 1 Present address, Massachusetts Institute of Technology, Cambridge, Mass.
860
ANALYTICAL CHEMISTRY
Jr., Tufts University, Medford, Mass.
crystal must be a t the center of rotation of the yoke and must also be immersed in an index of refraction liquid. The liquid is contained by capillary attraction in a cell which consists of two small pieces of microscope slide about 0.5 X 0.3 em. cemented to a slide about 2 mm. apart and covered by a fragment of a cover glass. If the center of rotation of the yoke is too high, the crystals may not be close enough to the condenser to give a good interference figure. A segment, G, of a protractor is mounted on the base beside the yoke
I
(
__---
which carries a pointer, H . A second segment, J , is mounted on an extension of the yoke. The pointer, I , on the shaft is mounted on a sleeve and may be positioned by means of the setscrew. J need cover only 180" because, if the pointer is properly set, both melatopes and both bisectrices of an interference figure can be brought to the cross hair without going off the scale. If the 180" scale is used, the side extension of the yoke need not be as long as illustrated, as it need not extend beyond the edge of the microscope stage. Brass stock inch thick was used for the base, B , and yoke; the axle mas made of '/Isinch drill rod, machined down to 0.030 inch a t the tip.
A similar instrument, lacbing the third axis of rotation, has been described by Wood and Ayliffe (S), who also describe a method for mounting crystals in a preferred orientation by means of a goniometer. This is seldom necessary with the device described here. Small crystals are simply "picked up'' from a flat surface by the tip of the shaft, which is previously touched to a tiny drop of Pliobond cement. The authors acknowledge the assistance of Andrew Levesque, who constructed the device and contributed materially to its design. I
Figure 1. Top and side views of rotation device and glass cell A . Baseplate B. Base C . Crystal mounted on shaft above cir-
cular aperture in base plate D. Pivots (setscrews with conical end) E. Shaft F. Yoke G. Segment of protractor H and I . Pointers J . Protractor K . Sleeve with set screw attached to pointer I L. Slots to accommodate stage clips M. Cell
LITERATURE CITED
(1) Federov, E. S., 2. Krist. 27, 337 ( 1897). (2) Gibb, T. R. P., Jr., "Optical Methods of Chemical Analysis," Chap. VI, McGraw-Hill, New York, 1942. ( 3 ) Wood, R. G., Ayliffe, S. H., J . Sci. In&. 12, 194 (1935).
EXCERPTED from a thesis submitted in partial fulfillment of the requirements for the B.S. degree at Tufts University. Contribution 237 from ,Department of Chemistry, Tufts University.
