TiCHBORNE — On Dissociation hy Heat of Compounds. 175 
The dissociation of water from lime seems peculiar to many of 
its compounds, and seems connected with the dismorphism of carbo- 
nate of calcium. See calcite and arragonite, p. 190. 
The determination of the solubility of lime at different tempera- 
tures are of importance in connexion with this report, because, inde- 
pendently of its geological bearing, it affords a simple but illustrative 
example of the dissociation of compounds generally when in solutiou. 
In this example we have the combined molecule H2O becoming 
separated in the presence of an overwhelming excess of water from the 
dissociative influence of heat. The gradual character of the action 
is also strikingly illustrated, the dissociation being in ratio to the 
increment of temperature, and is, under ordinary circumstances, only 
stopped by having arrived at the maximum temperature of water at 
the pressure of the atmosphere. 
The phenomena, however, is capable of further extension under 
increased pressure ad infinitum. The laws of molecular dissociation 
by heat have been only studied as far as regards volatile substances, 
but these laws seem applicable to the solution of substances modified 
by the altered physical condition, and are in their phenomena very 
similar. Thus, in speaking of gaseous dissociation, in connexion 
with the laws of Avogadro and Ampere, M. Wurtz"^' makes the follow- 
ing remarks : — 
''Un des meilleurs arguments qu'on puisse invoquer en faveur 
de cette interpretation est celui qui est tire de la densite de va- 
peur du Bromhydrate d'amylene. C'est une combinaison liquide 
du carbure d'hyclrogene amylene avec I'acide bromhydrique — Portee 
a une temperature pen superieure a son point d'ebullition cette 
combinaison montre une densite de vapeur qu'on pent appeler 
normale, parce qu'elle repond a 2 volumes de vapeur pour 1 mole- 
cule. La vapeur est intacte a cette temperature, mais qu'on la 
chauffe, elle va eprouver une decomposition plus ou moins com- 
plete, suivant que la chaleur fournie aura ete plus ou moins consi- 
derable .... Mais cette decomposition est graduelle ; elle ne 
s'acheve pas a un degre fixe, mais entre des limites de temperature 
assez etendues, de telle sorte que la vapeur, intacte a un certain degre, 
se trouve melangee, a des degres plus eleves, avec des portion de plus 
130° and tlien agitated, it soon becomes clear. 7000 grains of this water decanted 
require only 60 grains of test sulphuric acid. The same lime water was boiled with 
hydrate of lime for two or three mimites, and set aside to cool without agitation ; 
it very soon cleared — 7000 grains being decanted require only 46 grains of test sul- 
phuric acid to be neutralized, the test-acid being, as usual, 1-134. Hence we de- 
duce the following table : — 
One Part of Water, Takes up of Lime, Hydrate of Lime. 
60^ Faht., ^1? 
212° ,, Wto gi^l" 
* Histoire des Doctrines Chimique, par Ad. Wurtz, p. 79. 
