MOHAIR 359 



A mixture of bichromate of potash and boracic acid, in the proportion of eight equiva- 

 lents of crystallised boracic acid to one equivalent of the bichromate, or three parts crys- 

 tallised boracic acid to one part of the bichromate, is calcined at a red heat. Oxygen 

 and water are evolved, and amass is obtained, -which maybe regarded as a double salt 

 consisting of potassio-chromic borate. This is treated with water, which dissolves out 

 boracic acid and borate of potash, leaving an insoluble chromic hydrate, which, when 

 dried at a gentle heat and finely pulverised, constitutes the product in question. 



The boracic acid is, of course, recovered from the wash of waters by the ordinary 

 means for use in succeeding operations. 



The mode of printing with this powder is similar to that adopted in printing with 

 the ultramarines, albumen being generally employed as the fixing agent. Its use 

 was at the outset attended with considerable practical difficulty, which, however, 

 M. Kestner is stated to have now in a great measure overcome. The precise che- 

 mical constitution of this pigment is not yet fully elucidated, especially as to the 

 question whether or not it retains any portion of boracic acid. On this subject, M. 

 Guignet, after having described the process of its manufacture, observes that his 

 product, like the hydrated binoxides of chromium, is converted by heat, first into the 

 black binoxide, and subsequently (at a red heat) into the anhydrous sesquioxide. The 

 loss of water during this transformation appears, from the mean of three analyses, to 

 be 18-5 per cent. ; corresponding to the formula, Cr 2 3 . 2HO (Cr 2 O 3 . 2H-O), for tho 

 hydrate. This would show a proportion of water less than that of the ordinary hydrate, 

 of which it nevertheless presents the general character. On the other hand, M. Guignet 

 conceives that it may possibly retain traces of boracic acid, the presence of which, 

 however, he has found difficult to prove. Boracic acid, acting at a red heat upon 

 bichromate of potash, may, he remarks, produce simultaneously chromic and potassic 

 borate, or even a double compound of the two. Theoretically, this would yield to water 

 soluble potnssic borate, leaving behind only the insoluble hydrated oxide ; but, in 

 practice, a complete exhaustion of the mixed mass is hardly probable. 



M. Salvetat, who has studied the vert de Guignet, assumes the formation to be a 

 double compound of potassic borate and chromic borate. 



Finding that tho precise composition of this compound was thusj in fact, unknown, 

 and that no complete analysis of it was extant, Dr. Hofmann was led to request Mr. 

 Shipton, a young chemist working in his laboratory, to analyse the specimen exhibited 

 in 1862 by M. Kestner, and placed by that gentleman at Dr. Hofmann's disposal. 



The presence of boracic acid in this specimen was at once unmistakably indicated 

 by the appearance of the characteristic green-edged flame, when a portion of it was 

 exposed on a piece of platinum-foil, to the action of a strong red heat. 



Dried at 100 Cent, the substance lost a small percentage of water (from 7*43 to 

 7'46 per cent.), which loss was increased by ignition. As, however, tho determination 

 of water by ignition would, in this case, have been attended with uncertainty, seeing 

 that the partial transformation of the sesquioxide into the binoxide of chromium 

 would have caused the percentage of water expelled to appear less than it really is, 

 Mr. Shipton determined the water directly by strongly heating a portion, dried at 

 100 Cent, in a current of air, and collecting, in a chloride of calcium tube, the water 

 thus evolved. The chromium was converted by fusion into chromic acid, and the 

 latter determined in the form of chromate of lead. The boracic acid, lastly, was esti- 

 mated by difference as loss. 



Mr. Shipton arrived in this manner at tho following percentages : 



Composition of Vert de Guignet after having been dried at 100 Cent. 



I. II. Mean. 



Oxide of chromium . . . . . 76'39 76'56 76'47 



Boracic acid 11-89 12'30 12-10 



Water ....'... 1172 11-14 11-43 



100-00 100-00 100-00 



ASOCHA STOKE. See AGATE. 



MOHAIR is the hair of a goat which inhabits the mountains in the vicinity of 

 Angora, in Asia Minor. 



Wo are indebted for this account of mohair to the ' History of the Worsted Manu- 

 facture of England,' by James. 



Very much akin to, and in Yorkshire rising into importance about tho same time 

 as that of alpaca, the mohair manufacture demands attention. 



The goat is among the earliest animals domesticated by man, and undoubtedly, 

 from the very earliest ages, the fabrication of stuffs from its hair was practised by the 

 nations of antiquity. Throughout the middle ages the art of making beautiful stuffs 

 from the covering of the goat prevailed. 



