HOLM AN. — PYROMETRY. 239 



point an inch or less above the diaphragm. The sulphur should 

 extend from the bulb to a point half an inch or more into the tube. 

 It will usually be found necessary to pour out the liquid sulphur at 

 the close of a calibration to avoid breaking the tube at the next heat- 

 ing. If there is trouble from breaking of the tube during boiling 

 (none with good glass) a wire copper gauze wrapped about the bulb 

 will be found a preventive. The boiling should be maintained for a 

 few minutes after the sulphur vapor has risen above the top of the 

 asbestos to insure uniformity of temperature. It is advantageous to 

 put the wires through a small hood, or umbrella of asbestos at a 

 point about an inch above the junction, so shaped as to shed the drip 

 of cooler liquid sulphur which will run down the wires. A special 

 form of this hood is shown at G in the figure. Both this and the dia- 

 phragms in the tube are, however, unnecessary in ordinary work. 



For comparatively rough work it is sutficient to boil some sulphur 

 in an ordinary or small test tube (about one fourth full) and hold the 

 junction at the surface of the boiling liquid, taking pains not to have 

 it touch the tube, and also not to overheat the tube. This is the cus- 

 tomary procedure, but is not quite worthy of the instrument. 



The barometer and its temperature should be observed at the time 

 and place of making the calibration, although the errors from omis- 

 sion of this are negligible in rough work. 



Copper. — The high grades of commercial copper are good enough* 

 for this use ; such, for example, as are used in the best rolled sheet 

 copper and wire. This, of course, is easily obtainable and inexpen- 

 sive, and may probably be relied upon as having a melting point not 

 much, if any, over 1° C. below that of pure copper. Electrolytic copper 

 of a much higher grade of purity is preferable, and is not now rare or 

 expensive. 



A fire clay crucible,! 1^ to l.V inches deep, and about 1 inch in 

 diameter, is held in place inside a small furnace by pieces of fire-clay, 

 as shown on page 240. A double-walled furnace of the design and 

 proportions shown % (the figure is half size) is most satisfactory. It 

 is about 4^ inches outside diameter, and 3^ inches high. The cru- 

 cible contains upwards of fifty grams of copper. Above this, within 



* See results by H., L., and B. 



t Buffalo Dental Co., BuflEalo, N. Y. 



X Wiesiiegg, also Carpentier, botli of Paris, supply such a furnace ; or 

 one may readily be made by any manufacturers of fire-clay apparatus. The 

 Fletcher furnaces may also be used but are not so convenient for the purpose 

 because the products of combustion escape at the top. 



