M. TECHNOLOGY. 527 



of flexible tube conveying the gas, thus allowing movement 

 every way. It is scarcely necessary to state that, if the foil 

 is not in close contact with the hot plate at any part, then a 

 hole is sure to appear. This can not possibly occur where 

 the foil is held together by every particle resting on the 

 bed. 



The foils now only require to have the crystalline pattern 

 developed. This is done by immersing them for a few min- 

 utes in a mixture of equal parts of nitric and hydrochloric 

 acids diluted with five parts of water. As soon as the crys- 

 tals appear perfect, the foils are thoroughly washed, so as 

 to remove all traces of acid, and then coated with hard, 

 transparent varnish, which may be colored blue, red, yellow, 

 etc., according to fancy. These colors must, of course, be 

 transparent pigments, such as are well known to the varnish 

 manufacturers. 18 A, March 13, 1873, 629. 



EFFECT OF ALKALIES ON COTTON AND LINEN GOODS. 



According to Jeanmaire, cotton or linen fabrics which 

 have been impregnated with chromic acid, or a mixture of a 

 chromate and an acid, or permanganate of potash, and which 

 exhibit no perceptible change when washed, after the reduc- 

 tion of the chromic or permanganic acid has begun, are much 

 impaired in strength by treatment with any alkaline liquid, 

 even with soap ; although the injurious change in the fibre 

 will only be noticeable after some time, when the alkaline liq- 

 uid is very dilute. Acidity of the oxidizing liquid is not a 

 necessary condition, since the solution of permanganate of 

 potash acts in the same way, even when slightly alkaline, 

 and also a mixture of ferricyanide of potassium and alkali. 

 Injurious changes of linen, frequently noticed when it is 

 treated repeatedly with soap or lye, as well as unpleasant 

 experiences in bleaching, may result from the same cause. 

 Thus chromate of baryta or of lead, fixed on cloth, subse- 

 quently passed through an acid bath, may act in a similar 

 manner. This reaction also serves to show whether a white 

 or a yellow on indigo blue has been produced by means of a 

 resist, or by printing an acid on the cloth impregnated with 

 bichromate of potash; since, in the latter case, the white 

 portion will become rotten when it is dipped in an alkaline 

 solution. 6 C, November 5, 1873, 96. 



