36 Mr. S. U. Pickering on the Heat of 



attraction between the solvent and substance, even when the 

 latter is supposed to be dissociated, which attraction, when 

 satisfied, gives rise to a considerable evolution of heat. The 

 following determinations, however, will, I trust, possess some 

 interest beyond that of dismissing an already moribund theory. 

 Indeed a knowledge of the thermal phenomena of solution 

 must be of the highest importance for arriving at any true 

 theory on the subject, although, so far as I am aware, these 

 phenomena have up to the present been entirely overlooked 

 by the advocates of the physical theory. 



As substances for investigation , I confined my attention to 

 non-electrolytes, wdiere the results obtained would not be com- 

 plicated by any supposed dissociation j and, by selecting those 

 of which the heat of vaporization was known, it was only neces- 

 sary to determine their heat of dissolution when in the liquid 

 condition in order to arrive at their heat of dissolution in the 

 gaseous condition. Water, benzene, and acetic acid were 

 taken as solvents. The various results will be found in 

 Table I. Three determinations w r ere made consecutively in 

 the same quantity of solvent, so as to obtain the values for the 

 formation of solutions of three different strengths. The rise 

 or fall of temperature noticed on adding the three consecutive 

 quantities of x grams of the liquid is given under t' — t ; w is 

 the water-equivalent of the calorimeter with its contents; 

 D™ the heat of dissolution of a gram-molecular proportion of 

 the liquid in the solvent the composition of which is entered 

 in column II.; and t° C. the initial temperature of the deter- 

 mination. In the case of volatile liquids the determinations 

 are necessarily less accurate than in other cases, although the 

 error was reduced as far as possible by using a closed calori- 

 meter, and the experimental error may amount in extreme 

 cases to five times that obtaining with non-volatile liquids 

 (see Chem. Soc. Trans. 1890, p. 100), that is, to '005° on a 

 single determination representing an error of 50 cal. in the 

 value of D™ where water was the solvent, 30 cal. with acetic 

 acid, and 15 cal. with benzene. The temperatures of the 

 added liquid and of the solvent were nearly identical when 

 the two were mixed, and the heat-capacity of the solution 

 was taken as being equal to the sum of those of its consti- 

 tuents. The details as to the substances used are as follows: — 



Acetic Acid Mol. wt. =59-86. Sp. ht. =-46. P.p. =16-56° C. 



Benzene „ =7782. „ =436. „ = 544. 



Alcohol „ =45-9. „ =60. B.p. =7874 at 778 mm. 



Ether „ =7384. „ =-54. „ =34 53 at 759 mm. 



Acetone „ =57-87. „ = 52. „ =56'06 at 762 mm. 



Pyridine „ =7886. ( „ =-45?) „ =115-8 at 760 mm. 



Carbon Disulphide „ =73-89. „ =-24. „ =46'15at 7605mm. 



