120 Mr. Nicholson on the Composition of Caffein, 



II. 0*2500 grm. of substance gave '0744 grm. of silver. 



III. 0*2716 grm. of substance gave '0810 grm. of silver, 

 which give the following per-centages : — 



I. II. II. 



Carbon . . . 26*45 



Hydrogen . . 2*86 



Silver 29*76 29*82, 



and the formula — Cjs Hjo N4 O4 + AgO, NO5, 

 as may be seen by the following calculation : — 



The only analogues to this singular compound which I 

 know are those of urea and nitrate of silver, analysed by Wer- 

 ther* : the formulae of which are— Co H4 Ng Og + Ag O, NO5, 

 andC,H4N,0, + 2(AgO,N05). 



These compounds, however, in consequence of the peculiar 

 nature of xarea, are not very stable, being decomposed when 

 boiled with water into nitrate of ammonia and cyanate of 

 silver. There likewise exists a compound of nitrate of silver 

 and glycocoll, lately described by Horsfordfj having the for- 

 mula 



C4H4N03+AgO,N05; 

 and, according to H. Rose, a compound of nitrate of silver 

 with ammonia, 3 equivs. of this gas being absorbed by 1 

 equiv. of the former salt. 



Chloride of Mercury and Caffein. 



This beautiful compound is obtained w^hen an aqueous or 

 alcoholic solution of caffein is added to a solution of chloride 

 of mercury ; the latter being kept in excess, the fluid remains 

 perfectly clear, but after the lapse of a few seconds solidifies 

 into a mass of very small crystals, W'hich when recrystallized 

 from water or alcohol and washed on a filter, are quite pure. 



"When pure and crystallized from water it is very similar in 

 appearance to caffein, the crystals not being however quite so 

 large. It is very soluble in alcohol and water, hydrochloric, 

 nitric, and oxalic acids, and seems to form with the latter a 

 crystalline compound. It is nearly insoluble in aether. In 

 reference to its constitution, it is distinguished from the dou- 



* Liebig's Annalen, Ivi. 262, t Ibid. Ix. 36. 



