128 



An improvement on this process was suggested by M. Peligot, 

 who conveys the evolved ammonia into a quantity of sulphuric acid 

 of known strength, and then ascertains how much of it has been 

 neutralised, by saturating it after the operation with an alkali, a solu- 

 tion of lime and sugar being used for this purpose ; or, as Mr Mitchell 

 since then proposed, caustic soda solution may be substituted for the 

 lime with advantage, as it is not apt to change by being kept, either 

 from spontaneous decomposition, or the absorption of carbonic acid, 

 which indeed would not alter its neutralising power, nor affect the 

 delicacy of the operation, unless it took place to a very great extent. 

 This last-named method has the very great advantage of being quickly 

 performed, so that as many as six or eight analyses can be easily 

 made in the time required for one, according to the method of Var- 

 rentrapp and Will. The only difficulty lies in the preparation of the 

 solution of sulphuric acid, which must be done by guess, and the 

 proportion then ascertained by a baryta analysis, which is not a 

 very easy operation, from the finely-divided sulphate of baryta passing 

 readily through the filter. 



From these circumstances I was induced to try if the sulphuric 

 acid could not be replaced by some substance easily obtainable, of 

 definite compositions, in the solid form, not hygroscopic, and of which 

 the quantity could be easily determined by weighing, without being 

 liable to those sources of error which apply to sulphuric and most 

 other acids. The substance I have found to fulfil these conditions 

 is the anhydrous bisulphate of potash, a salt described by Jacquelain 

 in the Annales de Chimie et de Physique for 1839, but the exist- 

 ence of which seems to be doubted by many chemists. Into this 

 point I shall not at present enter particularly, as I have not yet 

 completed some observations upon this subject with which I have 

 lately been engaged ; but this has been ascertained, that if more than 

 two equivalents of sulphuric acid be added to any salt of potash con- 

 taining a volatile acid, and the mixture be exposed to heat at a certain 

 temperature, hydrated bisulphate of potash is formed ; and if the 

 temperature be now raised to incipient redness, in the dark, vapours 

 escape, and the anhydrous bisulphate, of perfectly definite composi- 

 tion, remains, which suflPers no farther alteration, even when the heat 

 is continued for so long a period as three hours — that is, if it be not 

 carried beyond incipient redness in the dark. 



In order to ascertain this latter point, I have had eight specimens 



