﻿the Combination of Acids and Bases. 33 



that the observed fall represented approximately its true amount. 

 When hydrochloric acid of equivalent strength was diluted to the 

 same extent, an elevation of temperature of o, 05 was produced. 

 The accuracy of experiments of this kind, where the whole 

 thermal effect observed amounts only to 2° or 3°, depends greatly 

 on the thermometer employed. Unless its indications are per- 

 fectly trustworthy in every part of the scale, the labour of the 

 inquirer will only end in disappointment. I have therefore taken 

 every precaution to secure this important object. The tube of 

 the thermometer was calibrated and divided with care, according 

 to an arbitrary scale, by means of a dividing-instrument contrived 

 for the purpose, and provided with a short screw of great accu- 

 racy, made by Troughton and Simms. The divisions, etched 

 finely on the glass, corresponded to about o, 05 C. ; and the 

 readings could be made with certainty to less than O- 01. The 

 division of the scale corresponding to 0° was determined from 

 time to time in the usual way ; and another point, about 30° C, 

 was fixed by comparison with four other thermometers similarly 

 constructed, whose scales extended from the freezing- to the 

 boiling-point of water. The readings of these four instruments, 

 when reduced to degrees, rarely differed from each other within 

 the limits to which they could be read, or 0°*02. The reservoir 

 of the thermometer used in these experiments was 75 millims. 

 long, and, when immersed in the liquid, occupied nearly its en- 

 tire depth. 



As some uncertainty always exists with regard to the thermal 

 equivalent of glass vessels, I made two sets of comparative expe- 

 riments — one with a thickly varnished copper vessel, and the 

 other with a vessel of platinum. The mean result of these expe- 

 riments coincided almost exactly with the result obtained when 

 the glass vessel was employed. 



The weight of the glass vessel which contained the alkaline 

 solution was 58 grms., and corresponded thermally to 11*4 

 grms. of the solutions formed. The thermal equivalent of the 

 reservoir of the thermometer and of the stirrer was 0*9 grm. 

 The alkaline solution weighed 160 grms., and contained the 

 equivalent of 1*738 grm. of SO 3 . The acid solution weighed 

 42*5 grms. Hence the entire thermal value of the apparatus, 

 in terms of the solution formed, was : — 



grms. 



Solution 202-5 



Glass vessel 11*4 



Thermometer and stirrer . 0*9 



214-8 



A correction (additive) of s ^ was made to the direct readings 

 Phil. Mag. S. 4. Vol. 41. No. 270. Jan. 1871. D 



