Dr. Stenhouse's Examination of Astringent Substances. 331 



The crystals of malate of lead are thus constituted : — 

 1 proportion of acid ... 58 ... 29*68 

 I proportion of oxide of lead 112 ... 56*62 

 3 proportions of water . . 27 ... 13*70 



100*00 

 100 grains of the crystals exposed in a thin stratum on a 

 porcelain dish, can lose at 212° 9*2 water = 2 proportions, 

 but it required to be heated in a thin glass tube, by means of 

 an oil-bath to 356° Fahr. before it lost the other third. When 

 the crystals are boiled in water, they lose also 2 proportions 

 of water, and assume the form of dough after it has been 

 kneaded ; the mass on cooling becomes as brittle as resin. 



XLI. Examination of Astringent Substances (continued). 

 By John Stenhouse, Esq., Ph.D.* 



Black and Green f^* REENand black tea are said by Mulder, 

 Tea. ^^ the chemist who has most recently ex- 



amined the subject, to be both derived from plants of the same 

 species. The differences observable in them are, as he al- 

 leges, chiefly owing to their being collected at different pe- 

 riods of their growth, and to the greater or less degree of 

 heat with which they are subsequently dried ; the black teas 

 being strongly heated upon iron plates, while the green teas 

 are exposed to a comparatively moderate temperature. If 

 this statement is correct, it may serve to explain what has been 

 long observed, that an aqueous infusion of black tea, though 

 quite transparent while hot, becomes muddy on cooling, 

 while an infusion of green tea retains its transparency even 

 when quite cold. The reason of this difference probably is, 

 that most of the essential oil of black tea is converted, by the 

 partial roasting it has undergone, into a resinous matter, which 

 though soluble in hot is nearly insoluble in cold water, while 

 the essential oil of green tea, on the contrary, remains nearly 

 unchanged, which is probably the cause both of the clearness 

 of its solution and perhaps also of the more powerful effect 

 which green tea is well known to exert on the animal ceco- 

 nomy. 



The aqueous infusion of both green and black tea give dull 

 olive-black precipitates with protosulphate of iron, which on 

 standing become leaden black. Infusions of tea also, when 

 evaporated to dryness and distilled, give crystals of theine 



* Communicated by the Chemical Society; having been read Feb. 21, 

 1843. The former part of Dr. Stenhouse's paper appeared in Phil. Mag., 

 S.3. vol.xxii. p. 417. 



