Chemical and Physical Notes 83 



should therefore be determined for the particular sample of 

 glass used in the construction of the thermometers. By 

 working in association with the thermometer-maker there 

 is no difficulty in ascertaining the exact weight of mercury 

 in the thermometer, or that of the glass which goes to the 

 bulb. Then if we multiply the weights of these substances 

 used by their respective specific heats, the sum of the products 

 is the thermal mass of the bulb expressed as the weight, in 

 grammes, of water, which is thermally equivalent to it. This 

 constant is usually, and conveniently, called the water value. 

 Calorimetry is an important department of physics and 

 physical chemistry, and the methods of determining water 

 values are given in all treatises on the subject. Where the 

 size of the thermometer is considerable the specific heat of 

 its bulb can be determined directly by the old "method of 

 mixtures," the thermometer itself being one party to the 

 mixture. As the term of cooling of a thermometer increases 

 in proportion to the size of the bulb, it is clear that thermo- 

 meters intended for meteorological use should have as small 

 bulbs as possible, and the method of mixtures is not applicable 

 for the determination of their thermal masses. Again, it is 

 out of the question to expect these thermometers to be 

 constructed so that the respective weights of mercury and 

 of glass in their bulbs shall be accurately known. The 

 following method of determining this constant is very con- 

 venient. It depends on mensuration, and was published by 

 the writer in I894 1 . 



Estimation of the Thermal Mass of the Bulb of a Thermo- 

 meter by Mensuration. The method is shortly stated in the 

 fallowing rule : Determine the external volume or displacement 

 of the bulb in cubic centimetres; multiply it by 0*475, an d the 

 product is fJie water value of the bulb in grammes. The factor 

 0-475 which occurs in this rule is arrived at as follows: The 

 density of mercury is 13*596, and its specific heat is 0*033, 

 therefore the capacity for heat of I c.c. is the same as that of 



1 ' On Rapid Variations of Atmospheric Temperature, especially during FHkn, 

 and the methods of observing them,' by J. Y. Buchanan, F.R.S. Proc. R. X 

 (1894), vol. Ivi. p. lift. 



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