OF ARTS AND SCIENCES. 87 



origin of the salt, gives a formula for its preparation, and then the 

 composition as determined by him. 



The receipt he gives is as follows, which he states works as well in 

 small amounts as in large : — 



"10 parts verdigris are mixed with as much water at 50-55° as is 

 necessary to produce a moderately thin emulsion : this is then passed 

 through a fine hair sieve, to separate the marc of the grape or the 

 small particles of copper from the basic acetate of copper. 



" This emulsion is poured still warm into a solution of 8 parts of 

 powilered arsenious acid in 100 parts water, which is kept in a state 

 of brisk ebullition in a copper kettle. The mixing is done quickly, 

 and meanwhile the fire must be kept bright and hot, so that the boiling 

 of the solution of arsenious acid may not be interrupted. The color 

 appears in a few minutes. If the verdigris is added cold, or the boiling 

 of the arsenious acid is interrupted, the precipitate is of a dull yellow- 

 green color, in which case some acetic acid must be added to the solu- 

 tion, boiling a few minutes longer, and allowed to stand to cool by itself, 

 after which the dull precipitate becomes crystalline, and is converted 

 into Schweinfurt green. 



" On account of the difficulty with which the arsenious acid dissolves 

 in pure water, many makers add to the water, with the 8 parts of ar- 

 senious acid, -^ parts (that is, to 8 pounds 1 oz.) of pure potash ; but, 

 before the addition of the verdigris, the solution must be made neutral 

 with acetic acid. . . . 



" If a boiling solution of neutral acetate of copper and one of arse- 

 nious acid in water, the amounts of each being equal, are mixed, a very 

 voluminous precipitate of a dull olive-green color occurs immediately : 

 if the supernatant liquid be allowed to stand in contact with this some 

 hours, or cool slowly with it, it loses its gelatinous condition, sinks 

 together, becomes crystalline in granular crystals, and assumes the use- 

 ful color of Vienna green. 



" The compound acquires a still more brilliant color, if an equal 

 volume of cold water is added to the liquid after mixing. 



" If the mixture, instead of standing by itself, be boiled a few min- 

 utes, the conversion to the crystalline salt is accomplished in a very 

 short space of time. 



'' The difference in the shades of color is due mainly to the size of 

 the crystalline grains : when rubbed to a fine powder on a color-slab, 

 their appearance is similar. . . . 



" The pure compound which was obtained by the preceding method 

 was submitted to analytical investigation. . . . 



