CHEMICAL AND PHYSICAL NOTES, 135 
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 1 ¢.c. is the same as 
that of 0°4486 grm. of water. The density of ordinary glass may be 
taken at 2°6 and its specific heat at 0°198, whence the capacity for 
heat of 1 c.c. of glass is the same as that of 0°5148 grm. water. There- 
fore when referred to unit volume the specific heats of these two bodies 
are very nearly identical, and if we have the total volume and apply the 
mean of the above values, the result will be a very close approximation 
to the water value of the bulb, quite independently of the exact pro- 
portion in which the mercury and the glass enter into its construction. 
Suppose that the displacement of the bulb were 1 cc. and that the 
glass were infinitely thin, so that the bulb were all mercury, then its 
water value would be 0°4486 grm. If, on the other hand, the internal 
volume of the bulb were infinitely small, so that it consisted entirely 
of glass, its water value would be 0°5148 grm. But the bulbs of 
thermometers of most ordinary patterns are very much alike in 
construction. It is not usual in the construction of even the best 
instruments to take into account the amount of mercury or of glass 
present in the bulb. Such data are available only in the case of 
thermometers especially constructed for use in calorimeters. As 
they are themselves parts ot the calorimeter, their heat constants 
must be ascertained; and, as was pointed out above, this is most 
easily and most accurately done during their construction. Ber- 
thelot * gives the thermal constants of three thermometers which 
were determined during construction. These are embodied in columns 
1, 2 and 3 of the following table. Column 4 refers to a thermometer 
belonging to the writer; it is by Chabaud, of Paris, and is constructed 
for use with Berthelot’s calorimeter. The data have been ascertained 
during construction, and are engraved upon the stem. 
In the following table the numerical data in the first part are 
taken from ‘ Berthelot’s Mechanique Chimique,’ pp. 162, 167. 
In this table the fundamental gravimetric data are—b the weight 
of mercury and d the weight of glass in the bulb—both of which are 
furnished by the makers, whose names are given in line a. In 
columns 1, 2 and 3 the water values of the mercury and the glass, ¢ 
and ¢, are given by Berthelot. In column 4 they were calculated, 
using 0°033 as the specific heat of mercury and 0°198 as that of 
glass, and the constant sought and arrived at in this part of the table 
is f, the water value of the bulb asa whole. The second part of the 
* ‘Essai de Mechanique chimique, fondeé sur la Thermochimie,’ par M. Berthelot. 
Membre de l'Institut, Paris. Dunod, 1879. 
