306 HEATS OF FORMATION OF THE CYANOGEN SERIES. 



surrounding atmosphere, which should not vary to any appre- 

 ciable extent during the whole course of the experiment We 

 then follow for half an hour the rate of cooling, which corre- 

 sponds to this fresh increase of temperature, the conditions 

 observed being, as nearly as possible, the same as those of the 

 first stage. 



The above experiment gave 



Initial temperature of the calorimeter ... +19*82 



Initial temperature of the enclosing vessel ... + 19'98 

 Final temperature of the enclosing vessel ... + 20'06 



Calculation from the experiment. We now have to calculate 

 the actual quantity of heat disengaged during this experiment. 

 It is obtained, as we know, by multiplying the masses em- 

 ployed, reduced to units of water by the variation of tempera- 

 ture observed, this variation increasing with the lowering of 

 temperature produced during cooling. 



Masses reduced to units of water. Of the substance em- 

 ployed, the mass existing at the end of the experiment consists 

 of that in the water, which contains about ? -^o its weight of 

 hydrochloric acid, goW ^ ammonium chloride, and about as 

 much formic acid. Their weight being known from the original 

 data, the density is next taken, and then the volume calculated. 

 We assume that 1 cc. of this liquid absorbs 1 cal. for a rise in 

 temperature of 1, an hypothesis sufficiently near the truth for 

 calculations of this kind. 1 



We reduce to units of water the various vessels of platinum 

 and glass that are used, and also the thermometer (that is, the 

 portion submerged), multiplying the weight of each vessel or 

 portion of vessel by its specific heat. The sum of all these 

 masses represents the total mass that has been subjected to the 

 variation of temperature observed. 



The actual variation of temperature is the apparent variation 

 plus that corresponding to the heat lost during the first and 

 second stages of the experiment. 



The calculation of these quantities will now be given, and 

 first of all, that during the second stage, as it is the easiest. 



Heat lost during the second stage. This is easily calculated, for 

 the duration of the second stage is only one minute, with a 

 final excess of temperature of 2 '5 above that of the enclosing 

 vessel. In fact, the loss of heat during the few following 

 minutes was measured and found to be almost uniform. We 

 calculate from this the mean loss during one minute ; then we 

 multiply the quantity thus got by the fraction f , for we assume 

 that the excess of temperature in the calorimeter, which 

 varied from 1*5 to 2*6 during one minute, has caused a loss 



1 " Annales de Chimie et de Physique," 4* se"rie, torn. xxix. p. 163. 



