ANALYSIS OF COMPLEX MATTER 119 



placing the crucible over a Bunsen flame. The solid liquefies, 

 and begins to give off brown fumes. If the crucible be now 

 removed from the flame, the change stops. It recommences 

 as soon as it is again brought into contact with the body at a 

 high temperature. After a time the observed change ceases, 

 and, instead of the white crystalline silver nitrate, we have a 

 white metal, silver. The quantity of silver is found by weigh- 

 ing. In order to make sure that the action is complete, the 

 whole should be raised to a somewhat higher temperature, and 

 again weighed after cooling. The quantity should be found 

 the same ; if not, the action was probably incomplete. The 

 process should be repeated until the mass is constant. It may 

 be noted that the liberated vapour diffuses into the atmo- 

 sphere, and it should be remembered that the silver nitrate 

 is exposed only to the pressure of the atmosphere while it 

 decomposes. 



Another quantity of silver nitrate is now weighed, and 

 similarly maintained at a high temperature. The quantity of 

 silver obtained will be found to bear the same ratio to the 

 quantity of silver nitrate used as the quantities in the previous 

 experiment. If further experiments are made the same 

 definite relations will be found, provided that no change but 

 the one in question has been going on i.e. provided the re- 

 quired experiment has been correctly performed. We may, 

 therefore, state that silver nitrate is probably a substance con- 

 taining always a fixed proportion of silver. Fuller examina- 

 tion would enable us to make more exact statements about its 

 composition. Changes of this class are called chemical changes. 

 78. Changes observable when Silver lodate is maintained 

 at a High Temperature. Weigh a small quantity of pure 

 silver iodate ] in a weighed crucible, and gradually raise the tem- 

 perature, and keep it at a moderate temperature until no further 

 change takes place. This point is ascertained by cooling and 

 weighing several times, until it is found that the quantity of 

 matter remaining in the crucible is constant. 



1 Potassium chlorate may be substituted for silver iodate, which is a 

 costly compound. The results, however, will be less satisfactory. 



