On the Combinations of Chromium. 143 



by an excess of sulphuric acid. When heated, its color 

 changes to dai'h orange-red, scarlet, and finally to red brown ; 

 on cooling it returns to its former color ; this change takes 

 place under water, and is common to several of the chro- 

 mates. It is obtained, by adding a solution of bichromate 

 of potash, to a solution of nitrate of lead, — a yellow powder 

 falls, which, by washing becomes orange yellow. 



Chromate of Silver. — In the form of powder, its color is 

 red brown ; it is instantly decomposed by chlorides and mu- 

 riatic acid. Sulphates also decompose it ; it is soluble in 

 ammonia, and when the solution is exposed to the air; long 

 pointed filaments, formed by the aggregation of acicular 

 prisms, are deposited. When a dilute solution contained 

 in a conical glass, is partly neutralized, by a few drops of ni- 

 tric acid introduced at the bottom, the crystals form in a few 

 hours, — they are often one inch in length.* It may be ob- 

 tained, by double decomposition. 



Bichromate of Silver. — Its crystals are rhombic tables, 

 and scales of an indeterminate form ; color red, with a tinge 

 of brown ; decomposed by chlorides, sulphates, and muriatic 

 and sulphuric acids; soluble in warm nitric acid, and is depos- 

 ited as the solution cools, in the form of rhombic tables. It is 

 obtained by dissolving chromate of silver in nitric acid ; or by 

 adding oxide of silver to chromic acid. It is this salt which 

 forms the ruby red crystals, described in our elementary works, 

 as crystals of chromic acid. When a slip of silver is dipped 

 into chromic acid, it is instantly acted upon, and crystals of 

 the bichromate are formed. This effect is sooner produced if 

 a drop of sulphuric acid, be added to the solution. If an arc, 

 composed of iron and silver, be used with this solution, the 

 silver is negative, with respect to the iron ; — but a galvano- 

 scope of sixteen turns of wire, when made the medium of 

 communication, does not indicate the existence of a current. 



Chromate of Mercury. — It exists in the form of a powder; 

 its color is dull yellow. It is partly decomposed by hot ni- 

 tric acid. Muriatic acid dissolves it, and forms bichloride of 



* Mi-. Teschemacher, has noticed the production and crystalization of this 

 salt, in the mixed solution of chromate and nitrate of potash ; — this salt is inso- 

 luble in an alkaline solution containing potash, and the crystals observed by 

 him, were probably the bichromate of silver. 



