[4] The thermochemical determinations for phosphorus and its compounds date from the last century, when Lavoisier and Laplace burnt phosphorus in oxygen in an ice calorimeter. Andrews, Despretz, Favre, and others have studied the same subject. The most accurate and complete data are due to Thomsen. To determine the heat of combustion of yellow phosphorus, Thomsen oxidised it in a calorimeter with iodic acid in the presence of water, and a mixture of phosphorous and phosphoric acids was thus formed (was not any hypophosphoric acid formed?--Salzer), and the iodic acid converted into hydriodic acid. It was first necessary to introduce two corrections into the calorimetric result obtained, one for the oxidation of the phosphorous into phosphoric acid, knowing their relative amounts by analysis, and the other for the deoxidation of the iodic acid. The result then obtained expresses the conversion of phosphorous into hydrated phosphoric acid. This must be corrected for the heat of solution of the hydrate in water, and for the heat of combination of the anhydride with water, before we can obtain the heat evolved in the reaction of P_{2} with O_{5} in the proportion for the formation of P_{2}O_{5}. It is natural that with so complex a method there is a possibility of many small errors, and the resultant figures will only present a certain degree of accuracy after repeated corrections by various methods. Of such a kind are the following figures determined by Thomsen, which we express in thousands of calories:--P_{2} + O_{5} = 370; P_{2} + O_{3} + 3H_{2}O = 400; P_{2} + O_{5} + a mass of water = 405. Hence we see that P_{2}O_{5} + 3H_{2}O = 30; 2PH_{3}O_{4} + an excess of water = 5. Experiment further showed that crystallised PH_{3}O_{4}, in dissolving in water, evolves 2·7 thousand calories, and that fused (39°) PH_{3}O_{4} evolves 5·2 thousand calories, whence the heat of fusion of H_{3}PO_{4} = 2·5 thousand calories. For phosphorous acid, H_{3}PO_{3}, Thomsen obtained P_{2} + O_{3} + 3H_{2}O = 250, and the solution of crystallised H_{3}PO_{3} in water = -0·13, and of fused H_{3}PO_{3} = +2·9. For hypophosphorous acid, H_{3}PO_{2}, the heats of solution are nearly the same (-0·17 and +2·1), and the heat of formation P_{2} + O + 3H_{2}O = 75; hence its conversion into 2H_{3}PO_{3} evolves 175 thousand calories, and the conversion of 2H_{3}PO_{3} into 2H_{3}PO_{4} = 150 thousand calories. For the sake of comparison we will take the combination of chlorine with phosphorus, also according to Thomsen, per 2 atoms of phosphorus, P_{2} + 3Cl_{2} = 151, P_{2} + 5Cl_{2} = 210 thousand calories. In their reaction on an excess of water (with the formation of a solution), 2PCl_{3} = 130, 2PCl_{5} = 247,