102 
PROFESSOR W. RAMSAY AND DR. S. YOUNG 
from both these objections, agrees closely with the point obtained by the barometer- 
tube method. This is shown by the large cross on the diagram. 
§ 54. Phthalic Acid, CgH^COOHjcj.—This acid melts at 184° (Lossen), decomposing 
into water and phthalic anhydride, CgH^CO^O. Ador (‘ Annalen,’ 163, p. 230) states 
that large crystals melt at 213°, while, when powdered, the melting-point is 203°. 
Pressures of Dissociation of Phthalic Acid in Barometer-tube. 
Temperature. 
Pressure. 
1 
Duration of experiment. 
o 
millims. 
/ 
30-8 
4-7 
19 
59-8 
20-8 
46 
78-0 
28-8 
62 
100-0 
41-2 
84 
116-0 
51-9 
140 
131-95 
61-7 
151 
131-6 
60-1 
125 
143-6 
111-9 
423 
149-9 
164'6 
384 
149-8 
163-1 
18 
154-9 
215-5 
363 
159-5 
327-3 
257 
164"6 
443-2 
142 
The tube was here refilled, as the original quantity of substance was deemed 
insufficient; the remaining numbers of this series will be referred to under succinic 
Temperature. 
Pressure. 
Duration of experiment. 
O 
millims. 
165-3 
514-3 
67' 
168-7 
629-6 
Constant 
Recombination 165’0 
518-7 
122' 
1681 
618-7 
118' 
The results of these experiments are graphically represented in Plate 2. The 
form of the curve appears to differ somewhat from that of an ordinary vapour- 
pressure curve, for the curvature is more abrupt on approaching the horizontal 
portion. This peculiarity will be referred to later. 
§ 55. Temperatures of Volatilization of Phthalic Acid. 
As phthalic acid dissociates on liquefying, the bulb of a thermometer was first 
covered with cotton-wool, and dipped repeatedly into a boiling aqueous solution of 
the acid. The thermometer was then hung in a bell-jar over sulphuric acid for several 
days until dry. The requisite temperature was obtained by jacketing with vapour of 
