Temperatures above the Boiling-point of Mercury. 425 



their entire length, the air being of course dried before it enters 

 the apparatus. The vapour of water, which otherwise obstinately 

 adheres to the glass in the capillary tube, is thus expelled. When 

 the entire length of the tube has been thus heated, the open end 

 of the air-reservoir B is closed and the whole allowed to cool. 



(3) The reservoir of mercury is contained in the bulb Q ; this 

 is fused to the end of the capillary stem, its other extremity being 

 shaped like the neck of a bottle. In order to avoid the intro- 

 duction of dust, this orifice is closed by a loosely fitting cork, just 

 sufficient to permit of the transmission of the atmospheric pres- 

 sure when the instrument is in use. The method of filling this 

 reservoir will be described presently. 



(4) The flat graduated scale, R R, is made of wood, and is 

 fixed along the entire length of the vertical portion //// of the 

 stem; it is consequently about 750 millims. in length. This 

 scale is held by the two small metallic clips p } p, which may be 

 tightened or loosened at pleasure by means of screws, so that it 

 may slide along the length of the stem 11 11. The scale is 

 further supported by the metallic branch A A, adapted to the 

 foot of the instrument. 



This scale is divided into two distinct systems of graduation; 

 the one to the left of the capillary tube is in millimetres, the di- 

 visions commencing from the top ; the other, to the right, is 

 divided into thermometric degrees, made empirically and by 

 means of certain fixed points which will be indicated presently. 



(5) The support. The thermometer is held by a firm and heavy 

 support. This support consists — 



1. Of a grooved shoulder, C C, upon which the horizontal 

 portion of the stem h h rests. 



2. At right angles to this shoulder and near one extremity is 

 soldered the metallic rod T T, in diameter about 5 or 6 millims. 

 This rod carries the horizontal branch A A, which has at its other 

 extremity a vertical slit to admit the wooden scale, R R. 



3. The rod itself is firmly fixed to the foot P P, sufficiently 

 heavy to give the requisite degree of stability to the whole ap- 

 paratus. 



II. Graduation of the Thermometer. 



This may be divided into three operations : (1) filling the in- 

 strument ; (2) determination of the fixed points ; (3) graduation 

 of the scale. 



1. Filling the Instrument. 



In order to fill the thermometer, a sufficient quantity of pure 

 dry mercury is poured into the bulb Q so as to half fill it ; a 

 partial vacuum is then made in the bulb by means of an air- 

 pump, the exhaustion being continued until the pressure is re- 



