58 MEASUREMENT OF HIGH TEMPERATURES. 
to the classical researches of Messrs. Deville and Troost. Availing 
ourselves of these, we adopt vapor baths, and in the present chapter 
uniformly adhere to them, all the temperatures chosen for calibration 
or definite comparison being boiling points. 
With the exception of certain forms of bath for boiling points below 
100°, our apparatus consists essentially of a closed crucible provided 
with an axial tube for the efflux of vapor. This tube projects inward 
or upward as far as the center of figure of the crucible, and downward 
through the bottom, of which it is a part. Distillation therefore takes 
place per descensum, through the tube, and the surface of the surround- 
ing boiling liquid is kept as nearly as possible at constant level as 
regards its position in the crucible. In the case of low boiling points 
this apparatus is made of metal and appropriately jacketed. For high 
boiling points, crucibles of graphite or of clay are preferable. In the 
interior of the central tube and near the center of figure of the cruci- 
ble the temperature is satisfactorily constant. Here, therefore, is 
placed the essential part or u explorer" of the pyrometer to be calibrated. 
It is thus exposed in the current of vapor circulating through the tube, 
the walls of which are permanently kept at the boiling point by the , 
boiling liquid surrounding them. In this way a comparatively simple 
form of apparatus, available at all temperatures, both high and low, 
retains all the essential features of the ordinary boiling point apparatus ; 
and it is only for very low temperatures ( < 100°) that a special form is 
expedient. 
Low boiling points. — To describe in passing this form of apparatus 
for low temperatures ( < 100°), we insert Fig. 1, in which the position 
of the thermoelement is indicated by t a T. The cold junction t is 
kept at constant and comparatively low temperature by water com- 
ing directly from the water mains and continually circulating around 
it* This cold water is further used in condensing the vapor after cir- 
culating around the hot junction T. A glance at the cut will make 
the disposition clear. Water enters at w and vapor at v, and after 
passing around the junctions t and T, respectively, they enter the con- 
denser, diagrammatically shown at cc, the water entering an external 
compartment and the vapor an internal compartment. The condensed 
vapor is at once refed into the boiler or flask, thus enabling the 
observer to use this apparatus quite as long as desirable without inter- 
ruption. At the cold end of the condenser the inner tube is in commu- 
nication with the air. Ebullition thus takes place under atmospheric 
pressure. The apparatus is available for experiments with ether, methyl 
alcohol, alcohol and water. It fails, so far as practical convenience is 
concerned, for aniline, etc., because of the difficulty encountered both 
in conveying the vapor into it from the boiler without condensation, 
and because of the corrosive action of such vapors on the corks and rub- 
ber tubing of the apparatus. If made of a single piece of glass it is 
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