﻿Intelligence and Miscellaneous Articles. 319 



its vapour, or should it be explained by the dissociation of the 

 elements ? 



We owe to M. H. St.-Claire Deville an ingenious experiment 

 which proves that in all cases the decomposition of sal-ammoniac is 

 not complete. Ammoniacal gas and hydrochloric acid gas heated 

 to 360°, and brought together in a space kept at this temperature, 

 would manifest their combination by a disengagement of heat which 

 would raise the thermometer beyond 390°. It is therefore impos- 

 sible to assume that in the vapour of sal-ammoniac at 360° the 

 elements are quite in a state of liberty. But it is not thereby proved 

 that at this temperature there is not a partial decomposition, which 

 may reach an amount sufficient both to explain the disengagement 

 of heat arising from the mixture of the two gases, and the low den- 

 sity of sal-ammoniac. 



Among the experiments which might throw some light on this point, 

 if not solve it, one of the most important would be a determination 

 of the latent heat of sal-ammoniac. For if this volatilization is due 

 to a simple change of state, it ought only to absorb a quantity of heat 

 analogous to that required to produce this same physical modification 

 in other compounds. If, on the contrary, it is accompanied by a 

 more or less complete chemical decomposition, it should require a 

 far greater quantity of heat, not greatly different from that which 

 results from the chemical combination of ammoniacal gas and hydro- 

 chloric acid. 



These considerations have led me to attempt this determination ; 

 but it presents such difficulties that I can only offer my results as a 

 rough approximation, sufficient, however, for the object in view. 



The method usually employed for the determination of the latent 

 heats of vapours is inapplicable in the present case ; for it is im- 

 possible to transport the vapour of sal-ammoniac from the vessel 

 where it is formed to the calorimeter. When it is no longer in con- 

 tact with surfaces heated to 350°, it condenses and soon stops up 

 even the widest tubes. 



I have endeavoured to invert the problem, and to measure the 

 quantity of heat used in volatilizing the salt in the open air, as com- 

 pared with that required for the volatilization of water under the 

 same circumstances. 



The apparatus I used consists of a massive cast-iron cylinder in 

 which three cavities are perforated symmetrically about the axis ; in 

 one of these is an air-thermometer, in the two others the substance 

 to be volatilized. 



The cylinder heated to redness is transferred to a box with badly 

 conducting sides, in such a manner that its upper face is exposed to 

 the air. 



The substance to be volatilized, contained in thin glass or silver 

 tubes, is placed in the cavities of the cylinder the moment it attains 

 a given temperature (500° for instance). The tubes are withdrawn 

 when the thermometer indicates 420° ; the loss of weight they have 

 experienced gives the amount volatilized. 



On the other hand, an investigation of the cooling of the appara- 

 tus, made by numerous experiments both when it oontained no sub- 



