404 Prof. Penny on the Composition and Phosphorescence 



The results of thesefewexperinientSj though affording very little 

 insight into the cause of the ]ihgenomena, plainly indicate that the 

 emission of light takes ])lacc within a limited range of tempera- 

 ture, and that actual contact of the crystals with the crystallizing 

 liquor is essential. Agitation of the liquor with a rod produces 

 no effect. 



Ill, Composition. 



I have already stated that the composition of this salt is 

 remarkable. It has hitherto been viewed and described as a 

 peculiar form or variety of neutral sulphate of potash, and as 

 such it is regarded both by chemists and manufacturers. The 

 results of my analyses do not confirm this view of its composition. 

 They show unequivocally, that, instead of being a mere cry- 

 stalline modification of the neutral sulphate, it is a double salt, 

 consisting of sulphate of potash and sulphate of soda. I have 

 analysed bold and well-defined crystals, carefully selected from 

 numerous specimens obtained from different makers, and I find 

 that they are perfectly uniform in composition. I have also 

 analysed crystals of the same salt, procured by recrystallizations 

 of the commercial salt. 



In these analyses it never gave more than 42*5 per cent, of 

 potash ; and parties acquainted with this article commercially, 

 know that in the bulk it never contains more than from 75 to 

 78 per cent, of sulphate of potash, — equal to about 41"5 per 

 cent, of potash, whereas sulphate of potash contains 54 per cent. 



It is not, I conceive, either desirable or necessary to give the 

 details of the methods of analysis, or particulars of the several 

 results. It will be sufficient to state, that the sulphuric acid 

 was determined, as usual, in the state of sulphate of baryta, the 

 potash by means of bichloride of platinum, and the soda indirectly 

 by deduction. The only precaution requiring notice is, that the 

 precipitated sulphate of baryta must be well washed after ignition, 

 to remove the potash which is carried down with the first pre- 

 cipitate. 



The mean of twelve analyses, the results of which accord very 

 closely, gave the following proportion of potash, sulphuric acid 

 and soda, in 100 parts : — 



Experiment. Theory. 



Potash .... 42-22 42-47 



Sulphuric acid . . 48-24 48-19 



Soda 9-54 9-34 



10000 100-00 



A carefully selected specimen gave 43-46 per cent, of potash. 



