174 Dr. Herapath on the Discovery of Quinine and Quinidine. 



right angles and without the stage. Immediately little circular 

 discs of white, with a well-defined black cross very vividly shown, 

 start into existence, should quinidine be present even in very 

 minute traces. Vide fig. 3. 



This is generally the case if hospital quinine or that of the 

 British Alkaloid Company has been employed — these severally 

 contain a very large per-centage of quinidine ; the former at 

 least 50, the latter about 20 ; but Howard's disulphate of qui- 

 nine scarcely contains 5 per cent, of quinidine, according to my 

 experiments. These substances are easily separated in conse- 

 quence of the much greater solubility of the disulphate of qui- 

 nidine in cold water. 



Water. 

 One part of disulphate of quinine requires . 740 at 60° ; 

 One part of disulphate of quinidine requires 340 at 55° ; 



so that the latter is more than twice as soluble. 



If we employ the selenite stage in the examination of this 

 object, we obtain one of the most gorgeous appearances in the 

 whole domain of the polarizing microscope; the black cross at 

 once disappears, and is replaced by one which consists of two 

 colours, being divided into a cross having a red and green fringe, 

 whilst the four intermediate sectors are of a gorgeous orange- 

 yellow. These appearances alter upon the revolution of the 

 analysing plate of tourmaline ; when the blue stage is employed, 

 the cross will assume a blue or a yellow hue according to the 

 position of the analysing plate. 



These phenomena are analogous to those exhibited by certain 

 circular crystals of boracic acid and to those circular discs of 

 salicine (prepared by fusion*), the difference being that the salts 

 of quinidine have more intense depolarizing powers than either 

 of the other substances, besides which the mode of preparation 

 effectually excludes these from consideration. Quinine prepared 

 in the same manner as the quinidine has a very different mode 

 of crystallization ; but it occasionally presents circular corneous 

 plates, also exhibiting the black cross and white sectors, but not 

 with one-tenth part of the brilliancy, which of course enables us 

 readily to discriminate the two. 



Having shown in my previous papers that none of the vege- 

 table alkaloids, when treated with sulphuric acid and iodine, 

 possess the power of forming crystalline compounds of similar 

 properties, and these artificial quinine tourmalines being pre- 

 eminent in their action on light, it follows that the existence of 

 these crystals is a positive demonstration of the presence of 

 quinine. It has also been proved by me that quinidine (/3 qui- 



* I am indebted to my friend Mr. John Thwaites for this fact. 



