MICROCHEMISTRY. 49 



ing to Borodin's method,* silver chloride may be tested with 

 a concentrated solution of silver chloride in concentrated 

 hydrochloric acid or salt solution (sodium chloride). 



II. Thallium sulphate causes at once, or at least on evap- 

 oration, the formation of regular octahedra or variously 

 shaped skeletons of thallium chloride, which may be tested, 

 according to Borodin's method, with a concentrated solu- 

 tion of thallium chloride. 



5. Sulphuric Acid, H a SO 4 , and its Salts. 



*J2. For the recognition of sulphuric acid we still lack a 

 completely trustworthy method. The following methods 

 have been used by Schimper (II, 219): 



1. Barium chloride always causes a precipitate of barium 

 sulphate, but this is rarely crystalline and its positive deter- 

 mination is therefore rarely possible. 



2. Strontium nitrate causes the formation of small, thick 

 crystals of a mostly roundish-rhombic form, though some- 

 times sharp and with straight outlines, which are insoluble 

 in water. 



3. Potassium sulphate often crystallizes out of a solution 

 of ash in water in the form of hexagonal plates, which fall 

 into colorless granules on the addition of barium chloride. 



* According to Borodin's method (II, 805) a given precipitate soluble in 

 water is tested with a completely saturated solution of the substance that is 

 suspected in it. If the suspicion is correct, the precipitate will not be dis- 

 solved, while any other substance, unless some reaction occurs, will be solu- 

 ble. If, for instance, we have to do with a mixture of asparagin and saltpeter 

 (potassium nitrate) the asparagin crystals will, of course, be insoluble in a con- 

 centrated solution of asparagin, but the saltpeter crystals will be dissolved. 

 On the subsequent addition of water, asparagin crystals will be dissolved also. 

 So, as in. the above-mentioned case, silver chloride will be insoluble in a con- 

 centrated solution of silver chloride in strong hydrochloric acid (or NaCl), 

 while it must dissolve on the addition of more acid (or NaCl solution). In 

 case of substances not too easily soluble, this method renders good service in 

 microchemistry ; but great care must be taken in each case that the solution 

 employed is really completely saturated, and that it does not become capable, 

 through changes of temperature, of dissolving more of the substance concerned. 



