512 Mr. S. U. Pickering on the Heat of Combination 



heats of fusion of the substances had therefore to be deter- 

 mined, and to reduce these to the temperature used in the 

 heats of dissolution determinations, the heat-capacity both in 

 the liquid and solid conditions had also to be determined. A 

 description of the method used and of the calculations will 

 be found in the Proc. Roy. Soc. xlix. p. 11 : it will be suf- 

 ficient to state here that the substance is heated in a platinum 

 bottle containing a thermometer to the required temperature, 

 and then plunged into the calorimeter. 



Table I. of the present communication gives the experi- 

 mental details and Table II. the results. Table III. gives 

 the values for the heat of dissolution — firstly, those obtained 

 by direct experiment on the substance in the one condition, 

 and, secondly, those calculated for it in the other condition, 

 as deduced from the former by means of the heat of fusion. 



The heat-capacity is calculated from the equation 



C _ (f-t)W-(T-<V 

 (r—t')w ' 



and the molecular heat of dissolution from the equation 



,_(*'-OWM. 



W 



where t' and t are the final and initial temperatures of the 

 calorimeter, W the water-equivalent of the calorimeter and 

 its contents, t the initial temperature of the substance when 

 introduced into the calorimeter, w its weight, and M its 

 molecular weight, uJ being the water-equivalent of the pla- 

 tinum bottle containing it. 



Considerable difficulty was experienced in finding sub- 

 stances suitable for the present investigation. In order that 

 the heat of fusion may be satisfactorily determined, it is 

 necessary that the compound and both of its constituents 

 should melt between temperatures of 0° and not much above 

 100°. Several substances other than those here mentioned 

 were examined and found unsuitable. 



Monohydrate of Sulphuric Acid. — The necessary data have 

 already been given in the Trans. Chem. Soc. 1890, p. 112, 

 and the Proc. Roy. Soc. xlix. p. 18 : a very slight alteration 

 in them has been made in consequence of round values for 

 the atomic weights having been used in the present work. 



Hexhydrate of Pinacone. — To prepare the anhydrous from 

 the hydrated pinacone supplied by Messrs. Kahlbaum, frac- 

 tional distillation was found to be unsatisfactory, and dehy- 

 dration by treatment of the ethereal solution with potassium 

 carbonate was adopted. The hydrated substance does not 

 appear to be by any means insoluble in ether, as is stated, and 



