and on a New Means of investigating them. 487 



which of course no longer interferes with the crystallization of 

 the herapathite. 



Method of applying the test. — After having ascertained in 

 the above mentioned manner that the test solution is absolutely 

 free from herapathite, a few drops of the liquid to be tested 

 are to be placed in a small porcelain basin, a little alcohol added 

 and then a few drops of test solution. When the quantity of 

 sulphuric acid is something more than a trace an immediate 

 black precipitate is formed. If the quantity is something less, 

 no immediate result is visible but as the alcohol evaporates, it 

 leaves behind a film of dark crystals which under a lens are 

 easily recognizable. In this way a quantity of sulphuric acid 

 amounting only to '000015 gram, about -gL of a milligram, can 

 be detected with certainty provided the test solution is not 

 used in too great excess. To detect so small a quantity how- 

 ever requires some familiarity with the reaction. 



With larger quantities of free sulphuric acid the character- 

 istic violet-black crystalline precipitate falls immediately. In 

 dealing with quantities too small to cause precipitation it is 

 advantageous to gently warm the basin beforehand ; in this 

 way a crystallization is obtained within two or three minutes 

 and the danger of secondary reactions such as might be caused 

 by the presence of oxidizing acids is diminished.* 



The conclusions derived from the foregoing investigations 

 may be briefly summed up as follows : — 



1. The solution of iodoquinia affords the means of detecting 

 free sulphuric acid even in traces in presence of combined 

 sulphuric acid. 



2. The salts of heavy metallic protoxides do not owe their 

 acid reaction to dissociation. The solutions of their sulphates 

 contain no free sulphuric acid. To this there is one exception. 

 Solutions of ferrous sulphate always contain free sulphuric 

 acid. 



3. Sesquisulphates as far as examined are always dissociated 

 in solution. 



4. Alums are always dissociated in solution with a. single 

 notable exception. Chrome alum exists as such in solution. 



5. Acid salts are dissociated in solution. The dissociation is 

 very great and may perhaps be complete. 



Philad., April, 1893. 



* Very faint traces of free sulphuric acid are detected as follows : If the test 

 liquid dries up without showing indications, 1 or 2 CC of alcohol are poured into 

 the basin and slowly washed round. As the yellow film dissolves it may leave 

 behind it a violet black film of herapathite which being much less soluble resists 

 the action of the alcohol longer. This is a test of very great delicacy and the 

 treatment with alcohol should never be omitted. 



