ON EVAPORATION AND DISSOCIATION. 
103 
bromonaphthalene, boiling at about 280°. The condenser was cooled by a freezing 
mixture. 
Pressure. 
Temperature. 
Pressure. 
Temperature. 
Pressure. 
Temperature. 
millims. 
O 
millims. 
O 
millims. 
O 
105 
191-9 
9-8 
201-7 
87-3 
196-9 
3'5 
198-4 
142-0 
210-1 
32-2 
168-6 
6'4 
199-4 
46-9 
177-1 
30-4 
202-9 
22-8 
159-5 
When the pressure was raised to 142 millims. the anhydride was left behind, and 
condensed as liquid on the sides of the tube, crystallizing on the colder parts. After 
the experiment, on removing the freezing mixture, the solid which had passed into 
the condenser liquefied partially, the liquid containing crystals. There had therefore 
been a separation of water. The solid in the condenser was evidently acid, and not 
anhydride, so that at very low pressures the acid had passed over undecomposed : it 
was then only after the pressure had been raised that decomposition took place, and 
it is probable that the substance had then melted, though it was difficult to observe 
this on the cotton-wool. The phthalic acid condensed in very small crystals, forming 
an opaque mass : the anhydride, in long transparent crystals. There was a distinct 
line of demarcation between the two. The last five numbers, which differ entirely 
from the others, are evidently the temperatures of volatilization of the anhydride, 
and the former ones of the acid. These numbers are reproduced in a separate curve. 
The temperatures of volatilization of the acid are plotted in Plate 2, and it is to be 
observed that, although there is an indication of a curve of the usual form, it com¬ 
pletely differs from that obtained by the other method. 
§ 56. Succinic Acid, C 3 H 4 (COOH) 3 .—This acid melts at 180°, and is said to boil at 
235° with formation of the anhydride, C 3 H 4 (C0) 3 0. The acid used in these experi¬ 
ments was recrystallized, and kept over phosphoric anhydride. 
Pressures of Dissociation of Succinic Acid in Barometer-tube. 
A very large number of experiments were made with this substance. In the first 
three series the numbers obtained gave, when graphically represented, curves with 
double flexure ; and so long as the same amount of substance was employed recombi¬ 
nation at any given temperature reached the same point as regards pressure as did 
decomposition. The curves obtained from these series were fairly concordant. But, 
on increasing the amount of substance, the pressures for given temperatures were 
much higher, and the double flexure tended to disappear. With phthalic acid it is 
possible that the vertical trend of the curve might, have been due to an insufficient 
amount of substance ; but this cannot have been the case with succinic acid. As the 
pressure of dissociation of succinic acid appears to depend on the amount of substance 
