| t | = | -28°·8 | -21° | -19° | -18° | |
| c | = | 84·2 | 86·8 | 92·6 | 98·4 | 101·4 |
| p | = | — | 334 | 580 | 900 | 1,073 mm. |
The last combination answers to the melted crystallo-hydrate HCl,2H2O, which splits up at temperatures above -17°·7, and at a constant atmospheric pressure when there are no crystals—
| t | = | -24° | -21° | -18° | -10° | -0° |
| c | = | 101·2 | 98·3 | 95·7 | 89·8 | 84·2 |
From these data it is seen that the hydrate HCl,2H2O can exist in a liquid state, which is not the case for the hydrates of carbonic and sulphurous anhydrides, chlorine, &c.
According to Marignac, the specific heat c of a solution HCl + mH2O (at about 30°, taking the specific heat of water = 1) is given by the expression—
C(36·5 + m18) = 18m - 28·39 + 140/m - 268/m2
if m be not less than 6·25. For example, for HCl + 25H2O, C = 0·877.
According to Thomsen's data, the amount of heat Q, expressed in thousands of calories, evolved in the solution of 36·5 grams of gaseous hydrochloric acid in mH2O or 18m grams of water is equal to—
| m | = | 2 | 4 | 10 | 50 | 400 |
| Q | = | 11·4 | 14·3 | 16·2 | 17·1 | 17·3 |
In these quantities the latent heat of liquefaction is included, which must be taken as 5–9 thousand calories per molecular quantity of hydrogen chloride.