HEAT OF VAPORISATION OP HYDROCYANIC ACID. 309 



SECOND STEP. 



2(H 2 + 0) = 2H 2 



H + C + N = HCN (pure and liquid) 



Reaction of HC1 (diluted) 



Sum ... + 149-15 

 Thus, 



x = - 149-15 + 126-55 = - 22'6. 



This value applies to liquid hydrocyanic acid. 



6. Vaporisation of hydrocyanic acid. In order to pass to the 

 gaseous state of the acid, we must determine the heat absorbed 

 in its vaporisation. To do this, the following method was 

 adopted, which may be applied to all liquids of similar volatility. 

 It consists in vaporising them in a current of dry gas and 

 measuring the heat absorbed. We pour into a glass phial a 

 known weight, say 1'396 grm., of pure hydrocyanic acid; we 

 then seal up the phial, which should be thin and provided with 

 two points easily broken. This is introduced into a little glass 

 receiver, fitted with a worm and arranged so that a regular 

 current of air may be made to circulate in it by means of an 

 aspirator, the gaseous current first passing through the recipient 

 and then through the worm. 



This little system is plunged into the calorimeter, which con- 

 tains 500 grms. of water. It is immersed almost up to the 

 orifice of the receiver, which is closed by a cork through which 

 a tube is passed, by means of which the current of air may pass. 



This air is perfectly dry, and its temperature during its 

 passage is shown by a thermometer indicating twentieths of a 

 degree; the volume of this air is determined sufficiently 

 accurately for the calculation into which it enters, by measuring 

 the volume of water that has flowed from the aspirator. It 

 may be added that a solution of alkali was placed between the 

 worm and the aspirator in order to absorb the hydrocyanic gas, 

 and thereby prevent its noxious fumes. These preparations 

 having been made, the phial being still closed, a certain volume 

 of air is allowed to circulate for twenty minutes through the 

 receiver and worm, in order to estimate the cooling. The 

 experiment for which the results are given gave a value of 

 for the initial cooling. This result is easily explained, as the 

 temperature of the water in the calorimeter was + 20'07, that 

 of the water of the enclosing vessel, + 20-22, and that of the 

 surrounding air, + 20'8. 



The phial is then broken against the sides of the receiver by 

 the violent shaking of the latter. The gaseous current is allowed 

 to continue circulating, and the thermometer is read. The 

 experiment lasts twenty minutes, during which the liquid acid 

 has entirely disappeared, and the minimum temperature is 

 reached almost immediately. This minimum corresponds to a 



