Heat evolved in Chemical Combination. 23 



examined and established when attention is directed to those 

 cases in which they act with but moderate activity; the results, 

 being less sudden and turbulent, become more easily subjected 

 to accurate measurement and observation. It has been re- 

 markably so with electricity ; and this it is which gives pecu- 

 liar value to the thermo-chemical researches recently entered 

 upon by Hess and Andrews. In Poggendorff's Annalen in 

 1839, Hess communicated some results on the admixture of 

 oil of vitriol and water, and announced that when two sub- 

 stances unite in many proportions, the quantities of heat evolved 

 in these acts of combination are multiple proportions of each 

 other. To the further development of this law he has since 

 devoted himself, and obtained a great number of important 

 results, which we will now proceed briefly to describe. 



The instances which first conducted M. Hess to the de- 

 tection of this law, were found in the quantity of heat evolved 

 by the combination of oil of vitriol with water. The method 

 first pursued was as follows : — The strength of the dilute sul- 

 phuric acid being determined, which did not evolve heat by 

 any further addition of water, and quantities of oil of vitriol 

 being diluted with water so as to bring them to various atomic 

 constitutions, these were added to the quantities of water 

 necessary to bring them to the standard first described, and 

 the rise of temperature consequent on the mixture accurately 

 determined. The two liquids (the water and atomically di- 

 luted oil of vitriol) had accurately the same temperature before 

 mixture, and the weights were accurately known. 



In order to compare the results given by a series of experi- 

 ments, some arbitrary unit should be taken, and for this was 

 selected one part of sulphuric acid considered as anhydrous; 

 for instance, if the experiment was made with S 3 + 3 H O. 

 Since the quantity of this last was known, the quantity of dry 

 sulphuric acid therein contained was also known, and let it be 

 marked 4. Then the heat evolved in the experiment, divided 

 by 4, gives the quantity of water which should have been 

 raised one degree of temperature by one part of anhydrous 

 acid. In this way the results which follow, and which were 

 announced by M. Hess in 1839, were obtained. 



Corrected 



The fourth column contains the number of units of heat 



