L. TECHNOLOGY. 445 



When the marble is perfectly dry it is to be moistened with 

 water, or exposed to moist air, when the decomposition of the 

 salt of iron referred to takes place in the upper strata, and 

 the process of coloration is complete. The surface can then 

 be polished, or, if already polished, it may be simply rubbed 

 off with a wet cloth. No injurious effect upon the polish or 

 hardness of the stone is produced by this operation. 



Our space does not permit us to give other applications of 

 this process, which can, it is said, be modified so as to produce 

 various shades of color, and promises to be of great value in 

 imparting to ordinary marble the appearance of certain rare 

 and costly kinds. 6 G\ December 15,496. 



TESTING THE PUEITY OP COMMEKCIAL INDIGO. 



As is well known, commercial indigo frequently contains 

 impurities, the percentage of which it is important to deter- 

 mine, these sometimes amounting to from twenty-five to sixty 

 per cent., and consisting principally of indigo brown, indigo 

 red, and indigo gluten, or gliadine. Of two methods appli- 

 cable in the case, the first consists in extracting the foreign 

 substance and leaving the pure indigo. For this purpose the 

 article is treated successively with potash lye, acetic acid, 

 and alcohol, the first extracting the indigo brown, the second 

 the gliadine, and the third the indigo red. The residuum is 

 then to be filtered, washed, dried, and weighed. The second 

 method is simpler and quicker in its action, and is intended 

 to determine the amount of impurities, which may be done 

 by dissolving out the indigo by means of smoking sulphuric 

 acid, which, however, takes up at the same time a little indi- 

 go brown and indigo red, and consequently rather too large 

 a percentage of indigo blue will be indicated. As the differ- 

 ence is but slight, the inaccuracy is counterbalanced by the 

 greater quickness of the process. The solution is to be made 

 in twenty times the weight of sulphuric acid, with 2000 parts 

 of water, filtered, dried, and weighed. 15 C, 1871,1,15. 



FACILITATING DYEING WITH INDIGO. 



The application of indigo in dyeing may, it is said, be 

 greatly facilitated, and with a great increase in the coloring 

 action of the indigo, by combining with each pound of the 

 indigo a mixture consisting of amorphous zinc, a pound of 



