Miscellanies. 409 



ance of the lamellar hydrate of magnesia of Hoboken, N. J. ; and 

 that of the aluminium has a bright sulphur yellow color, the crystals 

 standing out from the sides towards the centre of the tube in which 

 it was formed. Lt, Mather obtained about one ounce of each of 

 these chlorides at a single operation for each. — Extract of a letter, 

 of April 20, 1821, to the Editor. 



6. Pure chromate of potash. — -M. Zuber has published a process 

 for examining this salt, by means of tartaric acid, which is considered 

 as objectionable, on account of his selection of an acid, which is 

 rarely found in a pure state, and which forms with a solution of the 

 chromate, a very compound solution, in which the indications of re- 

 agents are not precise. 



It is better to use colorless nitric acid, adding about half a part to 

 the salt in solution, a drop of a solution of nitrate of baryta, will oc- 

 casion precipitation, if sulphuiic acid is present, or, a like quantity of 

 nitrate of silver, will cause the deposition of chloride of silver, should 

 muriatic acid contaminate it. Both chromates of baryta and silver, 

 are soluble, to a considerable extent, in chromic and nitric acids, and 

 if the reagents are not added in excess, there is no precipitation, if 

 pure chromate of potash is in solution. An exterjsion of this method 

 furnishes us with the salt in a state of purity. The chromate of 

 commerce is to be re-crystallized to remove the silicate of alumina 

 and oxide of iron, dissolved in water, nitric acid to displace one half 

 of the chromic is added and the liquor heated ; moist chromate of ba- 

 ryta may be dropped in, until the heavy sulphate ceases to fall, the li- 

 quor filtered, and chromate of silver mixed with it, so long as the curdy 

 chloride is formed, when it is again filtered and the clear liquor evapo- 

 rated to a salt and heated to redness in a platina dish ; the mass dis- 

 solved in water, affords crystals of the pure salt by slow evaporation. 

 It is impossible to remove sulphate of potash from this salt by the 

 processes of re-crystallization and solution. — A. A. Hayes. 



7. Covering for wires. — Mr. A. A. Hayes recommends mastic 

 colored by vermillion, or otherwise, as a very excellent substance for 

 covering the wires of galvanoscopes. Mr. H. melts it in a saucer 

 and slides the wire under the surface of the resin, by fastening a stick 

 across the saucer so as to have its edge dip ; then it is wound in a 

 coil or spiral. Its superiority is due to its tenacity when gently 

 warmed. 



