302 Prof. J. Joly ~on the Apophorometer. 



indifferent gas to be introduced around the heated substance 

 or a vacuum to be established. 



The procedure is as follows :-^The lower watch-glass is 

 raised till it is in contact with the ribbon. From 5 to 30 

 milligrams of the substance are distributed over the ribbon 

 so as to extend nearly over its entire length. The upper 

 glass is then laid on. All is now ready for the experiment. 

 In some cases it is desirable to weigh the platinum ribbon 

 before placing it in position. And, again, in some cases it 

 is advisable to make a preliminary experiment on a portion 

 (not weighed) of the powder in order to observe the be- 

 haviour of the substance. 



A current is now passed through the ribbon and gradually 

 increased. It may be that more than one sublimate is 

 obtainable from the substance by careful regulation of the 

 temperature ; or it may be that only the one is to be expected, 

 and the residue being non-volatile very great care in raising 

 the temperature is unnecessary. When the sublimate is 

 coming off freely the high temperature is maintained un- 

 altered for from 10 to 15 minutes or until the sublimation 

 is seen to be completed. The current is then cut off' 

 and the glasses let cool ; but while still considerably above 

 air-temperature the upper glass is lifted off, the lower one 

 let down and removed, and both glasses are clipped together 

 for weighing. This is important, as some sublimates are 

 hygroscopic. It is well to use watch-glasses which are 

 ground to meet accurately. 



I have assumed that the presence of oxygen is not injurious. 

 In many cases it is not. If it is, the receiver must be taken 

 into use and the air excluded. 



If there is any uncertainty as to the completion of the 

 process, a second pair of watch-glasses should be placed 

 around the ribbon and the current again turned on. We 

 can thus make sure that the volatile constituent has been 

 completely removed. 



The whole process is one of ease and simplicity. In most 

 cases the nature of the sublimate is anticipated, and in others 

 its character enables it to be determined ; and, of course, 

 chemical tests may be applied to it after its weight has been 

 ascertained. Knowing the nature of the sublimate, the 

 combined oxygen may be calculated out. 



The weight of residue left upon the ribbon is in some 

 instances of importance. Bat it is necessary to bear in 

 mind that the residual element may be oxidized in those 

 cases where access of oxygen has been permitted. It 

 will sometimes be found that from this cause the residue 



