Memoir upon Coffee. 25 



Among the vegetable acids, the malic, oxalic, and tarta- 

 rous, only gave, on their mixture w iih the acid of cofiee, 

 a clear veilow tint to the latter. 



The pure or saturated alkalis of potash, soda, and ammo- 

 nia, onlv gave as a product mixtures of a deep yellow in- 

 clining to orange. 



Lime, barytes, strontian, and the hydro-ulphurcts, pre- 

 sented, upon being mixed with this acid, phienomena little 

 dittercut from those of the -alkalis. 



Among saline substances tlie prussiate of amraonia gives 

 a product similar to that of the alkalis. 



The .prussiate of iron and potash develop a slight grccu 

 tint, which indicates the decomposition of the little prus- 

 siate of iron that this re-agent alwavs contains. 



Sulphate of alumine, acetite of barvtes, and the nitrate 

 of this latter base, arc partly precipitated in white powder 

 by this acid, after the mixture of these liquids has remained 

 some time at rest. 



Tannin, as well as gallated alcohol, are sensibly disco- 

 loured on mixture with the acid of coifee*. 



After having determined the chemical action of the acid of 

 coffee upon the difierent re-agents which we have here enu- 

 merated, it was necessary to inquire among the known ve- 

 getable acids for that which presented the greatest analogy 

 with the former acid. We first examined gallic acid, and 

 afterwards the tanning principle ; but we shall see, by com- 

 paring the most characteristic properties of these latter, that 

 wc cannot be permitted to establish an identity of a physical 

 or chemical nature between them and the acid Vvhich is the 

 iubject of our present inquiry. 



Tannin, very pure, precipitates in black the solution of 

 sulphate of oxygenated iron; the acid of coflce communi- 

 cates a green tint to it : the latter docs not precipitate aela- 

 tine ; wtide this is the most rcniarkable cliaracteristic of 

 tannin, and which would of itself suflficc to establish the 

 diflcrence which exists between these two bodies, if there 



* The precipitate of iyon obtalnfil from tlie solution of thp sulpliate of 

 th!i>baie witli tl)e acid of cofiee %v.(-. dissijlved in tlis nit/ic. 'iilpliiiric, phoi- 

 iilioiic, anJ ovi.Uc acijs., Tlie otlit-r .^^.•ItI^ ulil not ;iitai:k it Jl all. 



was 



