409 



using Paris green is the readiuess with which it settles to tlie bottom 

 of the tank or reservoir of the spraying apparatus. This is because 

 it is a rather coarse powder, very much less finely divided than Lon- 

 don purple, Avhich latter fact gives the purple a certain advantage. 



A recent conversation with one of the leading manufacturers of Paris 

 green has brought out the fact that this coarseness of grain and the 

 accompanying difficulty in using Paris green in water suspension are 

 quite unnecessary. It seems that the market has hitherto demanded 

 a very dark-colored article, the lighter color having been deemed to 

 indicate adulteration. It seems, however, that the darker color is 

 simply due to the larger size of the crystals, the darkest green having 

 the largest crystals. If the green were reduced to an impalpable powder, 

 which would make it much more satisfactory as an insecticide, it would 

 lose the intensity of its color and become whitish. This manufacturer 

 tells me that for a number of years he was able practically to control 

 the market, because he had discovered a means of crystallizing the 

 green in unusually large particles, securing a very dark-colored product, 

 although he was aware that for the use intended the product so obtained 

 was much less valuable. Yet the darker green, with its accompanying 

 larger-sized crystals, is more dilficult to manufacture, and hence more 

 expensive. 



It seems, therefore, that all these years we have been using a poison 

 which could have been had at less expense and in a much more satis- 

 factory form. 



Steps were therefore taken to secure a quantity of pulverized Paris 

 green which was made in the ordinary way, except that it was reduced 

 by the manufacturer to a nearly impalpable iiowder. The results were 

 eminently satisfactory, and would have been more so if the pulveriza- 

 tion had been carried to the fullest degree. This product remained in 

 suspension three times as long as the ordinary Paris green, and the 

 advantage of the fineness in division will undoubtedly be apparent in 

 its use against insects. 



It may be interesting to know the process of manufacture of this 

 arsenical. Powdered white arsenic (arsenious oxide) is combined with 

 sodium carbonate in a large vat of boiling water. A chemical change 

 results, in which the sodium unites with the arsenic, forming sodium 

 arsenite, and carbonic acid gas is given off. Sulphate of copi^er or 

 blue vitriol is dissolved in another large vat, and is then combined in a 

 still larger vat with the sodium arsenite already obtained. A precipi- 

 tate of arsenite of copi^er forms on the commingling of the two liquids 

 and sodium sulphate which remains in solution. Acetic acid is added 

 to the mixture thus obtained and unites with the precipitate of arsen- 

 ite of copper, forming an aceto-arsenite of copper, or more properly, a 

 double salt of copper with arsenious acid and acetic acid. This is 

 insoluble in water, and appears as a fine, green, crystalline product, 

 which, when properly dried, is ready for the market. 



