There are, therefore, three phases, viz. AgCl,3NH_{3}; 2AgCl,3NH_{3}, and NH_{3}, in the one case; and 2AgCl,3NH_{3}; AgCl, and NH_{3} in the other. These two systems are therefore univariant, and to each temperature there must correspond a definite pressure of dissociation, quite irrespective of the amounts of the phases present. Similarly, if, at constant temperature, the volume is increased (or if the ammonia which is evolved is pumped off), the pressure will remain constant so long as two solid phases, AgCl,3NH_{3} and 2AgCl,3NH_{3}, are present, _i.e._ until the compound richer in ammonia is completely decomposed, when there will be a sudden fall in the pressure to the value corresponding to the system 2AgCl,3NH_{3}--AgCl--NH_{3}. The pressure will again remain constant at constant temperature, until all the ammonia has been pumped off, when there will again be a sudden fall in the pressure to that of the system formed by solid silver chloride in contact with its vapour. The reverse changes take place when the pressure of the ammonia is gradually increased. If the volume is continuously diminished, the pressure will first increase until it has reached a certain value; the compound 2AgCl,3NH_{3} can then be formed, and the pressure will now remain constant until all the silver chloride has disappeared. The pressure will again rise, until it has reached the value at which the compound AgCl,3NH_{3} can be formed, when it will again remain constant until the complete disappearance of the lower compound. _There is no gradual change of pressure_ on passing from one system to another; but the changes are abrupt, as is demanded by the Phase Rule, and as experiment has conclusively proved.[152] The dissociation pressures of the two compounds of silver {84} chloride and ammonia, as determined by Isambert,[153] are given in the following table:-- -------------------------+------------------------- | AgCl,3NH_{3}. | 2AgCl,3NH_{3}. -------------+-----------+--------------+---------- | | | Temperature. | Pressure. | Temperature. | Pressure. -------------+-----------+--------------+---------- | | | 0° | 29.3 cm. | 20.0° | 9.3 cm. 10.6° | 50.5 " | 31.0° | 12.5 " 17.5° | 65.5 " | 47.0° | 26.8 " 24.0° | 93.7 " | 58.5° | 52.8 " 28.0° | 135.5 " | 69.0° | 78.6 " 34.2° | 171.3 " | 71.5° | 94.6 " 48.5° | 241.4 " | 77.5° | 119.8 " 51.5° | 413.2 " | 83.5° | 159.3 " 54.0° | 464.1 " | 86.1° | 181.3 " | | 88.5° | 201.3 " -------------+-----------+--------------+----------