432 PRINCIPLES OF CHEMISTRY 



chloride or sodium nitrate. The following are the data from which 

 Thomsen deduced this conclusion. He mixed any one of these three 

 neutral salts with an acid which is n.ot contained in it ; for instance, he 

 mixed a solution of sodium sulphate with a solution of nitric acid and 

 determined the number of heat units then absorbed. An absorption of 

 heat ensued because a normal salt was taken in the first instance, and 

 the mixture of all the above normal salts with acid produced an absorp- 

 tion of heat. The amount of heat absorbed enabled him to obtain an 

 insight into the process taking place in this intermixture, for sulphuric 

 acid added to sodium sulphate absorbs a considerable quantity of heat, 

 whilst hydrochloric and nitric acids absorb a very small amount of heat 

 in this case. By mixing an equivalent of sodium sulphate with various 

 numbers of equivalents of nitric acid Thomsen observed that the 

 amount of heat absorbed increases more and more as the amount of 

 nitric acid was increased ; thus when HN0 3 was taken per JNa 2 SO 4 , 

 1752 heat units were absorbed per equivalent of soda contained in the 

 jsodium sulphate. When twice as much nitric acid was taken 2026 

 . heat units, and when three times as much 2050 heat units were ab- 

 sorbed. Had the double decomposition been complete in this case when 

 one equivalent of nitric acid was taken, then the heat evolved would 

 be determined by the sum of 13617 15689 1650/1'S, or would equal 

 2989 heat units, if it be admitted that sulphuric acid when mixed 

 with NaNO 3 absorbs as much heat as when mixed with iNa 2 SO 4 . 

 But as in reality only 1752 heat units were absorbed instead of 2989 

 units, therefore a displacement of only about two- thirds of the sulphuric 

 acid had taken place that is, the ratio k : k' for the reaction 

 ^Na 2 SO 4 + HNO 3 and NaNO 3 -f ^H. 2 SO 4 is equal, as for ethereal salts, 

 to 4. By taking this figure and admitting the above supposition, 

 Thomsen found that for all mixtures of sodium with nitric acid, and of 

 -sodium nitrate with sulphuric acid, the amounts of heat followed 

 Guldberg and Waage's law ; that is, the limit of decomposition reached 

 was greater the greater the mass of acid added. The relation of hydro- 

 chloric to sulphuric acid gave the same results. Thus on mixing 

 4 with HC1, the thermal result of experiment was 1682, and 

 calculation 1690 ; when mixed with 2HC1 the experimental result 

 1878 and by calculation 1870, with 4HC1 the result of experi- 

 ment was - 1 896 and by calculation 1 9 1 7. When NaCl + |H 2 SO 4 was 

 taken, then experiment showed an evolution of + 244 heat units, whilst 

 from calculation it should have be'en +257 heat units; on doubling 

 the quantity of acid experiment gave +336 and calculation +292. 

 The slight differences between the results obtained by experiment and 

 calculation are due to the unavoidable errors of calorimetric determi- 



