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VII. On the Determination of the Heat- Capacity of a Ther- 

 mometer. By J. W. Clark, Assistant Professor of Physics 

 in University College, Liverpool*. 



I. TN the determination of specific heats a correction should 

 JL be made for that part of the thermometer which is 

 immersed in the water of the calorimeter, as its specific heat 

 is not the same as that of an equal volume of w r ater. Very 

 often this correction is reduced to a mere estimation on 

 account of the unknown weights of glass and mercury which 

 constitute the immersed portion of the thermometer. 



When two metals of known specific gravity are fused 

 together, and the volume of the resulting alloy is the sum of 

 the volumes of its two constituents, it is only necessary to 

 know the specific gravity and volume of the piece of alloy 

 to calculate the exact volumes of the metals comprising it. 

 Similarly, by determining the specific gravity and volume of 

 the thermometer, the volume of mercury which it contains can 

 be at once determined from 



where V 1 is the required volume of mercury in the thermometer, 

 V the volume of the thermometer, 

 S the specific gravity of the thermometer, 

 S 2 „ „ „ thermometer-glass, 



Si „ „ ofHg. 



The mean value of several very closely agreeing determi- 

 nations of the specific gravity of different specimens of ther- 

 mometer-glass is 3*199 for lead-glass and 2*512 for soda-glass. 

 Should it not be known of which sort of glass the thermo- 

 meter consists, it may be readily ascertained by slowly intro- 

 ducing the upper extremity of the instrument into the re- 

 ducing-flame of a blowpipe : — Soda-glass yields a yellow- 

 flame, but lead-glass blackens, from the reduction of the oxide 

 of lead which it contains. It may be assumed that thermo- 

 meters made on the continent consist of soda-glass ; those 

 made in England are usually constructed of lead-glass. 



The total volume of the thermometer (V) is obtained 

 from its weight in air and in water. 



The volume of mercury contained in the thermometer having 

 been found by (1), the volume of that part of the thermometer 

 which is immersed in the water of the calorimeter has next to 



* Communicated by the Physical Society : read April 25, 1885. 



