310 Progress of Foreign Science. 



till no more carbonic acid was expelled. He mingled, by 

 degrees, this hot solution with the first, agitating constantly till 

 the effervescence ceased ; a dirty greenish-yellow precipitate 

 fell in abundance. To this he added about three parts of acetic 

 acid, (three parts of which saturated 0.45 of carbonate of lime,) 

 or such a quantity as that there was a slight excess of it, per- 

 ceptible to the smell after the mixture. The precipitate gra- 

 dually diminished in size ; and, at the end of some hours, there 

 •was deposited spontaneously at the bottom of the liquor (now 

 colourless) a powder, somewhat crystalline, and of a fine green 

 colour. He separated the supernatant liquid, which, by resting 

 longer on the colour, might deposit oxide of arsenic, which 

 would render it paler. He afterwards treated it with a large 

 quantity of boiling water, to remove the last portions of arsenic, 

 beyond what existed in combination. We must take care not 

 to add to the solution of sulphate of copper an excess of 

 arsenite of potash, because it would saturate, in mere waste, 

 the acetic acid, which ought to be in slight excess in the mix- 

 ture, without causing any very obvious effervescence in it. 

 For this reason, it is proper, in general, to take a neutral 

 arsenite of potash. It is true that a portion of the arsenious 

 acid remains in the mother liquor ; but this may be employed 

 for the preparation of Scheele's green, commonly used for painted 

 papers of an inferior quality. It appeared that, when M, Bra- 

 connot added to the mixture, before the fine green colour was 

 pronounced, a small quantity of the latter ready formed, the 

 production of it was more speedily promoted, — as a crystal, 

 plunged in a saline solution, attracts the molecules similar 

 to its own. 



The process now described has been repeated on the great 

 scale, and with some modifications, at the manufacture of 

 M. Noel. An arsenite of potash was employed, which had been 

 prepared with eight parts of oxide of arsenic instead of six. 

 The liquors were concentrated. Some hours after the mixture, 

 a pellicle, of a very rich green colour, formed at the surface. 

 The whole being exposed to heat, a heavy powder fell down, 

 which was washed with abundance of water, to free it from the 

 excess of arsenious acid. The green thus obtained was magni- 

 ficent; and several unprejudiced colourists judged it to be 

 more powerful than that of Schweinfurt. — Ann. de Chim. et 

 de Phys., xxi. 53. 



8. On the Combinations of Chromic Acid with Potash. 

 By M. F. Tassaert,/Zs. 



This gentleman affirms that a solution of chromate of potash, 

 whether neutral or alkaline, will not yield crystals of a neutral 

 salt, which salt can exist only in solution ; and that, in reality, 

 the lemon-yellow salt, known in commerce under the name of 



