176 Scientific Intelligence. 



fluoric acid. The chromic acid, after being separated from the sparingly 

 soluble fluo-silicate of potash by filtration, is evaporated to dryness in a 

 platinum capsule, and then rcdissolved in the smallest possible quantity of 

 water. By this means the last portions of fluo-silicate of potash are ren- 

 dered insoluble, and the pure chromic acid is then separated by decanta- 

 tion. The acid must not be filtered in this concentrated state, as it then 

 corrodes the paper like sulphuric acid, and is converted into the chromate 

 of the green oxide of chromium. When it is wished to prepare a large 

 quantity of chromic acid by this process, porcelain vessels may be safely 

 employed in the first part of the operation, provided care is taken to add a 

 quantity of silicated fluoric acid, not quite sufficient for precipitating the 

 whole of the potash. When the evaporation has proceeded so far that the 

 liquid may be conveniently contained in a vessel of platinum, the silicated 

 fluoric acid is added in excess. 



M. Maus recommends, that in preparing the silicated fluoric acid the 

 usual materials should be placed in a capacious retort, the beak of which 

 descends into a large receiver with a long neck, without any luting. At 

 the bottom of the receiver is placed water for absorbing the fluo-silicic gas, 

 and its sides and neck are likewise moistened. On issuing from the retort, 

 the gas descends like a cloud upon the surface of the water beneath, where 

 the greater part of it is dissolved ; and any portions which escape are ab- 

 sorbed by the moisture on the sides of the vessel. By operating in this 

 manner very little gas escapes into the air, and at the same time, as the 

 beak of the retort does not touch the water, there is no chance of the aper- 

 ture being closed by the separation of gelatinous silica. 



21. On the detection of potash by the blowpipe, by means of the oxide of 

 Nickel. By M. E. Harkort. — In the ninth volume of PoggendorfFs 

 Annalen der Physik und Chemie, M.E. Harkort of Frey berg has described 

 a method of detecting the presence of potash in salts or minerals by means 

 of the blowpipe, and Berzelius recommends the method as decisive. The 

 remarks on the subject which Berzelius proposes introducing into the 

 next edition of his Treatise on the use of the Blowpipe are the following : 

 ( Poggendorff, xi. 333. ) ' ' I have found Harkort's method of detecting po- 

 tash to be wonderfully delicate. It is only necessary to dissolve oxide of 

 nickel in glass of borax, and then to add a little saltpetre, felspar, or any 

 substance containing potash, in order to procure immediately a very dis- 

 tinct blue glass, even when a small quantity of the alkali is present. 

 The presence of soda does not prevent this appearance." 



" Of the preparations of nickel, either the nitrate or oxalate may be em- 

 ployed. The latter is easily procured in a solid state, and on that account 

 is preferable for many experiments ; but the former is more convenient 

 for detecting potash in solution. It is essential to employ a salt or oxide 

 of nickel which is free from cobalt, — a fact easily ascertained, by the pure 

 nickel forming not a bluish, but a brown glass with borax. The blue co- 

 lour which the oxide of nickel forms with potash is different from that 

 produced by cobalt, and has the same purplish tint as the ammoniacal solu- 

 tion of the oxide of nickel by candle-light. 



