TjYONS — Vblnme Change on Fusion. 65 



The upper part of tlie bulb is drawn out to a short and very tine capillary, 

 the top of which is at first open and must lie at a lower level tlian that of 

 the horizontal stem. The flat form of the bulb is convenient for examining 

 the clianges in appearance of the substance in its stages of fusion, and has 

 the advantages of rapidly taking up the temperature of the bath. The 

 cylindrical form is, however, less likely to be affected by changes in pressure, 

 and is preferable where an exact determination at some definite temperature 

 is required. 



The stem is calibrated, and tlie mass of mercury required to fill one 

 centimetre at the temperature of the room is found. It will be convenient 

 to refer to this mass as {a). 



The apparatus with suitable means of suspending it from the balance is 

 weighed empty at first. This weight will be called m',. 



The bulb is next filled with mercury by connecting the end of the stem 

 to a small liand exhaust-pump, the open tip of the capillary above the bulb 

 being placed under the surface of clean mercury. When the apparatus is 

 filled, the pump is detaelied, the bulb is placed erect, and the mercury 

 adjusted so as to fill the stem to a convenient position. By carefully drawing 

 a bunseu flame across the tip of the capillary, the bulb is quite easily sealed. 

 The apparatus thus filled with mercury is weighed (W2). The mass of 

 mercury in the bulb [id - n\) will be referred to as (^o-) 



The apparatus is now adjusted, so that the bulb and bent portion of the 

 stem is immersed in a bath of water, glycerine, or oil, and the stem is fixed 

 horizontally in contact with a millimetre-scale etched on mirror-glass. A 

 mark X on the stem is put to a fixed division on the scale, so as to make the 

 scale-readings always correspond to the same point on the stem. The 

 bath is heated and the temperature regulated by a very sensitive xylol or 

 toluol thermostat. The liquid is kept well stirred by two small propellers 

 driven by a little electric motor. The bath is heated and cooled through 

 a range, such as will include the temperatures for which the apparatus will 

 be subsequently employed. The reading of the meniscus is observed at the 

 different temperatures by means of a low-power lens. The reading in the 

 case of this initial expansion of tlie mercury in glass will be referred to 

 as (i?o). 



To introduce the substance into the bulb, the capillary at A must he 

 opened by breaking off the point or fusing the top, when the internal pressure 

 of the mercury will make an opening. This may be effected so as not to 

 appreciably alter the weight of the apparatus and the mass of mercury. We 

 shall assume, however, that under the most unfavourable conditions the 

 capillary is broken off, and if necessary drawn out again. The apparatus 



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