NOTE TO PEREIRA ON LITMUS. 45 



plant and its locality, and mention its employment both in isso. 

 medicine and for dyeing. It is also described by Pomet, and 

 figured in the English translation of his History of Drugs, 

 printed in London in 1737. Pliny alludes to it under the name 

 of Heliotropion tricoccon. 



In conclusion, it may be observed as a curious fact, that 

 although formerly in general demand, Turnsole rags appear to 

 have fallen into complete disuse everywhere but in Holland, 

 in which country all that are now produced are consumed. Of 

 the uses to which they are applied by the Dutch we are still 

 in want of more precise information. 



Note addressed to Pereira on Dutch Cake Litmus. 



LONDON, 7th of 12th mo., 1850. 



MY DEAR DR. PEREIRA, Some months ago I was favoured Samples of 

 by thee with the sight of a specimen of litmus said to be free Litmus. 

 from indigo, which thou hadst recently received from Holland, 

 It was in much larger cakes than the ordinary kind, and of 

 not nearly so fine and deep a blue colour. This induced me to 

 examine two samples of litmus received by our house from 

 Amsterdam in 1846, one of which agreed in its characters with 

 that above-mentioned, while the other was of the usual sort, 

 and of excellent quality. 



1. The first of these, which I have since called litmus without 

 indigo, afforded no crystals of indigo on being carefully heated. 

 Its aqueous solution was of the usual intense purple, and the 

 residuum when deprived of all the colour cold water would 

 extract, was of a fine deep blue colour. 



2. The second sample, litmus containing indigo, on being 

 heated in a capsule, afforded the small copper-coloured crystals 

 so characteristic of indigo. The aqueous solution possessed nearly 

 the same amount of colour as the other, and of almost the same 

 tint. The residuum was also deep blue. Neither Prussian blue 

 nor cobalt could be detected in either sample. 



The remarkable point, however, in the history of these samples 

 is, that the value of the litmus in large cakes (No. 1) is so much 

 greater than that in small cakes (No. 2) that the difference in 

 favour of the former amounts to more than 50 per cent. 



It is no easy matter to say what is the cause of this singular 

 variation in price, as the cheaper litmus is of "better appearance, 

 and affords an amount of colour of similar richness, very nearly 



