242 SCIENCE IN SHORT CHAPTERS. 



are all divisible by eleven. The observed period of sun-spot occur- 

 rence is eleven years and a small fraction. 



I am anxiously awaiting the arrival of Lord Lindsay's long- 

 exposure photographs of the corona, for if they represent the varying 

 degrees of splendor of this solar appendage, the explanations ot- 

 fered in Chapter xii. of my essay on " The 1'uel of the Sun" will be 

 very severely tested by them. Yours respectfully, 



W. MATTIEU WTLTTAMS. 

 Woodside Green, Croydou, January 4, 1871. 



CHAPTER XXX. 



THE COLOEING OF GKEEN TEA. 



THE following is a copy of my report to the Grocer on a sample of 

 the ingredients actually used by the Chinese for coloring of tea, 

 which sample was sent to the Grocer office by a reliable correspondent 

 at Shanghai (November, 1873). I reprint it because the subject has 

 a general interest and is commonly misunderstood : 



" 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 connect- 

 ed with the history of the manufacture of Prussian blue. Messrs. 

 Bramwell, of 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 Ib. ; in 1815 it had fallen to 10s. 

 6d, in 1820 to 2s. 6cZ., then down to Is. 9d in 1850. I see by the 

 Price Current of the Oil Trade Review that the price has recently been 

 somewhat higher. 



" In the early days of the trade a large portion of Messrs. Bramwell'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 some- 



