224 SCIENCE IN SHORT CHAPTERS. 



I have examined the blue and the yellowish-white 

 powders received from the office, and find that the blue is 

 not indigo, as your Shanghai correspondent very naturally 

 supposes, but is an ordinary commercial sample of Prussian 

 blue. It is not so bright as some of our English samples, 

 and by mere casual observation may easily be mistaken for 

 indigo. Prussian blue is a well-known compound of iron, 

 cyanogen, and potassium. Commercial samples usually con- 

 tain a little clayey or other earthy impurities, which is the 

 case with this Chinese sample. There are two kinds of 

 Prussian blue the insoluble, and the basic or soluble. 

 The Chinese sample is insoluble. 



This is important, seeing that we do not eat our tea- 

 leaves, but merely drink an infusion of them; and thus even 

 the very small quantity which faces the tea-leaf remains 

 with the spent leaves, and is not swallowed by the tea- 

 drinker, who therefore need have no fear of being poisoned 

 by this ornamental adulterant. 



Its insolubility is obvious, from the fact that green tea 

 does not give a blue infusion, which would be the case if 

 the Prussian .blue were dissolved. 



There are some curious facts bearing on this subject and 

 connected with the history of the manufacture of Prussian 

 blue. Messrs. BramwellTof Newcastle-on-Tyne, who may 

 be called the fathers of this branch of industry, established 

 their works about a century ago. It was first sold at two 

 guineas per lb.; in 1815 it had fallen to 10s. 6d., in 1820 to 

 "2s. 6d., then down to Is. 9(7. in 1850. I see by the Price 

 Current of the Oil Trade Review that the price has re- 

 cently been somewhat higher. 



In the early days of the trade a large portion of Messrs. 

 Bramweli's produce was exported to China, The Chinese 

 then appear to have been the best customers of the British 

 manufacturers of this article. Presently, however, the 

 Chinese demand entirely ceased, and it was discovered that 

 a common Chinese sailor, who had learned something of 

 the importation of this pigmet to his native country, came 

 to England in an East Indiaman, visited, or more probably 

 obtained employment at a Prussian blue manufactory, 

 learned the process, and, on his return to China, started 



