186 OUTIJNES OF PHYSICAL CHEMISTRY
In the same way
H + CI = HC1 + 22 Cal. grams of gaseous hydrochloric acid.
Julius Thomsen has devised a very expressive method of writing thermo-chemical equations. He inserts a comma between the symbols of the reacting substances and encloses them in a bracket.
(H, CI) = 220 C. Heat of formation of gaseous HC1.
in much water. (H, CI, aq) = 39*4 C. Heat of formation of dissolved HC1. (H*, O) = 58*3 C. ; of water in the vapour state. 69*0 C. ; of liquid water (about 18°). 70-4 C. ; of solid water (at 0°). (NaOH aq, HC1 aq,) = 13*7 C. Heat of neutralisation of caustic soda
by hydrochloric acid in dilute solu- tion.
��APPARATUS AND METHODS
Thermometry
The sensitiveness of a thermometer depends on the relative capacities of the reservoir and the capillary stem.
For calorimetric use the thermometer must be very sensitive and graduated in fiftieths or hundredths of a degree. The exactitude of the instrument is always merely relative, and even the best thermometers require to be provided with a table of corrections. In order to draw up this table, the readings of the thermometer to be corrected may be compared with those of an air-thermo- meter. Thermometers which have been corrected at official institutes (such as the Physikalisch-technische Beichsanstalt at Berlin-Charlottenburg) can now be bought.
The direct verification of the zero point is best carried out by the cryoscopic method in Beckmann's apparatus.
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