EXPEEIMENTAL KNOWLEDGE OF THE PfiOPEETIES OF MATTER. 117 



Le Chatelier ^ gives a list of dissociation-pressures, with the tempera- 

 tures corresponding, as follows : 

 Temperature 



547° 

 610° 

 625° 

 740° 

 745° 

 810° 

 812° 

 865° 



for several varieties of calcium carbonate from different sources, all 

 agreeing to give the same dissociation-pressure throughout for each tem- 

 perature as soon as equilibrium had been reached, which was more rapidly 

 done the more finely divided the calcium carbonate. 



At about 812° the dissociation-pressure was equal to the atmospheric 

 pressure ; on heating rapidly, however, the temperature rose higher, up 

 to 925°, and stood for some time constantly at that point on account of 

 the rapid consumption of heat by the decomposition of the calcium carbo- 

 nate. Analogous results were obtained by Le Chatelier in the decompo- 

 sition of gypsum and of calcium hydrate ; results easily explained by the 

 length of time taken by bodies undergoing dissociation in reaching their 

 state of equilibrium. These higher dissociation tempei-atures suggest a 

 somewhat similar explanation for cases of overheating such as occur with 

 mercury and other bodies in which there is a difficulty in overcoming 

 cohesion or capillary action.^ 



Froofhj Direct Experiment that Curves of Vapour-pressure from Solid 



and Liquid are different. 



The difficulty of the problem in the case of water arises from the small- 

 ness of the pressure of vapour of ice even at 0°, viz., 4-6 mm., while for ice 

 at — 17'1° the pressure is 1-04 mm. It is easy to see that by any ordinary 

 m.anometric contrivance it would be difficult to get very satisfactory 

 results for such very low pressures; however, Pettersson in 1881 ^ suc- 

 ceeded by this means in getting a few results for temperatures at very 

 small pressures by using a thermometer surrounded with ice, a manometer, 

 and a 4-litre exhausted flask surrounded by a freezing mixture ; an 

 arrangement by which the ice round the thermometer distilled without 

 melting, while the manometric pressure corresponding to each temperature 

 of the ice could be observed. Some of the results were in fair accordance 

 ■with some data given by Regnault for vapour-pressures given by ice at 

 different temperatures below 0°. 



By this method, as Pettersson points out, the pressure continuously 

 and rapidly changes as the temperature of the ice rapidly rises ; the ther- 

 mometer is therefore not to be expected to indicate the temperature for 

 each pressure accurately, seeing that the ice and the mercury cannot take 

 up the new temperature corresponding to each new pressure instan- 

 taneously. 



» Compt. Rend. cii. 124.3. 



^ Horstmann in Berichte der Deutsclien Chemischen G. six Ref. p. 429. 



' Berichte der DevtscUen Chemischen G. 14a, p. 1370, 



