Page:Outlines of Physical Chemistry - 1899.djvu/158

 140 OUTLINES OF PHYSICAL CHEMISTRY

In thermo-chemical calculations we have constantly to take into consideration the water equivalent of the instru- ments, or in other terms, their caloric capacity. For the calorimeter, the stirrer, &c., the water equivalent is equal to the weights of these multiplied by their specific heats. The caloric capacity of the thermometer would be difficult to calculate were it not for the favourable circumstance that a cubic centimetre of glass has nearly the same water equivalent as a cubic centimetre of mercury :

2-5 x 0-190 = 0-47 for glass 18-6 x 0-084 = 0-46 for mercury.

It is, therefore, sufficient to determine the volume of the immersed part of the thermometer. This volume expressed in cubic centimetres, multiplied by 0*46, gives the water equivalent of the instrument.

Modus operandi. — All reactions do not lend them- selves to thermo-chemical investigation. This method is only applicable to certain kinds of quick transformations, which are finished after a few minutes, such as the neutralisation of acids by bases in aqueous solution, a large number of double decompositions between salt solutions, certain phenomena of solution, and of dilution, &c. The heat of combustion has been determined for a large number of organic substances and will form the subject of a later chapter.

In order to determine the heat of solution of a substance in the liquid contained in the calorimeter, the substance and the liquid are brought as nearly as possible to the same temperature, then the dissolving process is allowed to take place. By means of the stirrer, an equal distribu- tion of the materials and of heat through the whole mass is attained. The thermometer immersed in the dissolving liquid indicates the changes of temperature.

If it is required to study the action between two liquids or two solutions, one of them is placed in the calorimeter, and the other in a glass flask, or in a platinum