M. TECHNOLOGY. 509 



nese to the protoxide, the salts of which are readily removed 

 by subsequent washing. If the yarns or fabrics resist the 

 bleaching process they should be treated with hydrochloric 

 acid containing one part of commercial acid to twenty parts 

 of water. One advantage of this process is that it affords a 

 means by which even indigo may be discharged by a series 

 of successive bleachings, leaving the stuff fit for redyeing, 

 3 A, December 16, 1871, 473. 



BLEACHING BY PERMANGANATE OF POTASH. 



The German Dyers' Gazette highly recommends the bleach- 

 ing of cotton fabrics with permanganate of potassa. It as- 

 serts that the process is simple and speedy, and much less 

 injurious to the fibre than the chlorine bleaching, and that 

 whoever has given it a fair trial will never return to the old 

 method. The yarn is boiled in caustic lye for from four to 

 five hours, well rinsed, and for a quarter of an hour moved 

 about in a bath containing three per cent, of the permanga- 

 nate. It is then wrung and introduced into water to which 

 from four to five per cent, of sulphurous acid has been added. 

 Light rinsing, bluing, and drying finish the process. 26 C, 

 1872, vii. 



USE OF CHROMATE OF POTASH IN DISCHARGING COLORS. 



It often happens that when a mixture of gum dextrine with 

 chromate of potash is used for discharging colors, washing 

 will not leave the pattern white. This arises from the fact 

 that such a mixture, when exposed to light, becomes partly 

 insoluble, and, consequently, can not be entirely removed 

 from the cloth. The remedy is to work in rooms having yel- 

 low glass windows, which prevent the action of the chemical 

 rays of light, and allow the desired effect to be accomplished. 

 21 A, December, 1871, 1223. 



PIGMENTS AND DYES USED BY THE ANCIENTS. 



From a memoir by M. Rousset upon the pigments and dyes 

 used by the ancients, it would appear that the variety was 

 very considerable. Among the white colors, they were ac- 

 quainted with white lead; and for the blacks, various kinds of 

 charcoal and soot were used. Animal skins were dyed black 

 with nut-galls and sulphate of iron. Brown pigments they 



