ON EVAPORATION AND DISSOCIATION. 
89 
method, which will afterwards be given, and as this was not the case with other 
dissociating substances, the highest reading was repeated. 
§ 39. The vapour-pressure of mercury was redetermined in the same manner as 
before; the tube was jacketed with bromonaphthalene at a pressure of 752’2 millims. ; 
the temperature corresponding was 280°. The vapour-pressure was found, after the 
necessary corrections, to be 156 millims. After cooling, the pressure due to air was 
1*7 millim. ; this, corrected as before, gives a pressure of 0'6 millim. at the higher tem¬ 
perature. The pressure of the mercury-vapour is therefore 155'4 millims. From 
Regnault’s numbers, 155 ‘2 is the pressure at this temperature. # 
This tube, without being emptied, was boiled out again in vacuo, and some 
ammonium chloride, prepared from pure hydrochloric acid and ammonia, and pre¬ 
viously strongly heated, was introduced with the usual precautions. The tube was 
again jacketed with bromonaphthalene vapour, boiling at a pressure of 755'7 millims., 
corresponding to the temperature 280'1°; the pressure read was 145 - 4 millims., a 
number agreeing very closely with those previously obtained, viz., 144 - 4 and 
145'9 millims. 
§ 40. When the tube cooled, a permanent gas remained, which exerted a pressure 
of 18’3 millims. This amounts to a pressure at the higher reading of 4 - 2 millims. As 
a very long time was allowed so as to make certain of no further rise when the 
ammonium chloride was heated, the presence of this gas cannot be accounted for by a 
gradual decomposition of the ammonia, for we have shown (Chem. Soc. J., 1884, p. 88) 
that, ammonia begins to decompose only at about 500°, even when hydrochloric acid 
is absent. It might, however, be due to some extent to the action of hydrochloric 
acid on mercury. On inclining the tube, this gas formed a bubble of considerable size 
at the top. Water was then introduced, and almost complete absorption occurred. 
It appears certain, then, that gaseous hydrogen chloride and ammonia, when perfectly 
dry, combine very slowly. In any case, the error, owing to presence of permanent 
gas, amounted to only 4'2 millims. 
§ 41. It was deemed advisable to ascertain the vapour-pressure of ammonium 
chloride at still higher temperatures, and for that purpose the ordinary arrangement 
was inadmissible. We, therefore, adopted a method, the principle of which will be 
seen from the accompanying diagram. A is a tube, at the closed end of which a 
mass of ammonium chloride was placed above the constriction at B. This was done 
before the tube had been bent. Mercury was then introduced, occupying the level 
shown. This tube was surrounded by a jacket, C, containing a little mercury ; and 
was closed by a tight-fitting indiarubber cork, D, through which the open end of the 
tube, A, projected. The tube A was connected by means of lead-tubing with a 
differential gauge, E, behind which stood a graduated mirror scale, and with a force 
and exhaust-pump, G, so that pressure on the mercury in A could be increased or 
diminished to a known amount at wfill. The pressure exerted on the mercury in the 
* From our revised table, 157'35 millims. 
MDCCCLXXX VI. xr 
