A Versatile Microscope Hot Stage WILLIAM A. BONNER Stanford University, California
T
HE ease with which thermal behavior of substances may be studied microscopically has given rize to an extensive literature (1, 2, 3) describing the construction and use of hot and cold stages for the microscope. A number of the hot stage designs are specifically for the determination of melting points, and the authors claim accuracy and control comparable to or better than that attained by the customary heating-bath methods. The advantages of employing a microscope for melting point dkterminations, especially in conjunction with the use of polarized light, are summarized by Chamot and Mason (1). Such points in favor, as the. use of sma1ler.quantities of material, the increased ease in recognizing and distinguishing accompanying thermal behavior, tlle readiness with which impurities are detected, and the wider range of temperatures available, argue for a more extensive use of the microscopic method. The majority of the hot stage designs previously proposed, while possessing many desirable features, have several disadvantages in common. In the first place, due to a complete enclosure of the sample within the stage walls, there is no provision for removing or introducing samples after the stage is heated. Secondly, for the same reason, there are no means of examining any but a single portion of the field during heating. This is a distinct disadvantage, since the most significant 'portions of a melt are not necessarily in the field of the objective a t the outset. Lastly, the cover glasses upon which samples are usually placed are less convenient to manipulate than slides. A need on the part of the author for a more versatile hot stage has prompted a design which eliminates these disadvantages while retaining earlier desirable features.
SIDE
The present design utilizes an electrically heated metal block provided with a square heating chamber in the center large enough to accommodate an ordinary slide. Visual access to the chamber is provided by glass windows on the top and bottom of the stage. A horizontal hole in one end of the stage permits the insertion of a thermometer into the heating chamber, and a horizontal slit in the.other end provides for introduction, withdrawal, or manipulation of a slide in the chamber a t any time during the operation of the stage. The sample being studied is contained between a slide and cover glass. The stage is constructed entirely of brass according to the dimensions (millimeters) of Figure 1. The center block, containing the heating chamber, consists of a single piece, with the exception of a thin bridge, 2 mm. thick, screwed into place under the 2-mm. slide slit. Holes are drilled in the 3-mm. top and bottom pieces and recessed around the circumference for the insertion of glass windows. The top and bottom are then screwed to the center block, forming a single unit. A piece of one-eighth inch Transite board, having the dimensions of one end of the unit and drilled with a 7mm. hole for insertion of the thermometer, is screwed to the thermometer end of the stage. The Transite is drilled and countersunk for the attachment of binding post screws and serves both as a nonconducting anchor for the binding posts and as insulation for one end of the stage. Sixteen-millimeter circular glass windows (polarimeter windows) are secured in their recessed retainers in the top and bottom of the stage by means of a pulp made of asbestos and water. The entire unit is wrapped with damp asbestos tape, dried, wound with ten feet of No. 28 Chrome1 A resistance wire, then covered with six layers of damp asbestos tape. After
drying, the slide slit is readily cut open with a razor. Before assembly the walls of the heating chamber are blackened with lampblack to increase internal radiation (1). The assembled unit is advantageously painted with a mixture of aluminum bronze and sodium silicate solution. The present stage was designed for use with an inexpensive student microscope and is attached by unscrewing the stage of the latter and bolting the hot stage in position in its place. The heating unit is operated from a 5-ampere Variac transformer, and it is convenient to calibrate the Variac settings in terms of the stage temperatures. A temperature of 350° was attained with the Variac set a t GO. The thermometer is inserted into the heating chamber to the point where it just begins to appear in the field. The sample to be examined is placed on one end of a slide, pressed under a cover glass, and cautiously inserted into the slit so as not to cause the cover glass to slip. By employing both ends, a single slide suffices for two observations. Microscopically determined melting points are generally low due to the ease with which incipient fusion is observed (I), and it is therefore desirable to calibrate the stage by means of a series of pure samples of known melting point. The author is grateful to Mr. A. Jansse for aid in the construction of the hot stage described. LITERATURE CITED
CHAMOT. E. M., AND C. W. MASON,"Handbook of Chemical
Microscopy," 2nd ed., Vol. I, John Wiley and Sons, Inc., New York, 1938, pp. 19&209. SAYLOR, C. P., in "Temperature, Its Measurement and Control in Science and Industry," Reinhold Publishing Corporation. New York, 1941, pp.873-81. JELLEY, E. E., in "Physical Methods of Organic Chemistry," Vol. I, lnterkience Publishers, Inc., New York, 1945, pp. 445-7.
