Note 50 — Chapter XX · SULPHUR, SELENIUM, AND TELLURIUM

The amount of heat developed by the mixture of sulphuric acid with water is expressed in the diagram on p. 77, Volume I., by the middle curve, whose abscissæ are the percentage amounts of acid (H2SO4) in the resultant solution, and ordinates the number of units of heat corresponding with the formation of 100 cubic centimetres of the solution (at 18°). The calculations on which the curve is designed are based on Thomsen's determinations, which show that 98 grams or a molecular amount of sulphuric acid, in combining with m molecules of water (that is, with m=18 grams of water), develop the following number of units of heat, R:—

m = 1 2 3 5 9 19 49 100 200

R = 6379 9418 11137 13108 14952 16256 16684 16859 17066

c = 0·432 0·470 0·500 0·576 0·701 0·821 0·914 0·954 0·975

T = 127° 149° 146° 121° 82° 45° 19° 9° 5°

c stands for the specific heat of H₂SO₄mH₂O (according to Marignac and Pfaundler), and T for the rise in temperature which proceeds from the mixture of H₂SO₄ with mH₂O. The diagram shows that contraction and rise of temperature proceed almost parallel with each other.