Memoir upon Coff-ee. 309 



of volatile aromatic oil ; and the second, in order that the 

 external air rtiav not carrv off" too speedily the caloric from 

 the waier, and that it iiiav dissolve more easily the principles 

 with which it niav saturate its'.if in passing over the coffee. 



If, after what \vc have said above, we compare the liquor 

 proceeding from this last operation with that which results 

 from an equal quantity of water and coffee boiled together, 

 we shall find that the infused liquor also possesses a perfect 

 transparency, is of a very agreeable flavour, of a sweet taste, 

 and that it possesses, in short, all the properties which we 

 require in this beverage. 



The decoction, on the contrary, which is not aromatic, 

 and is disagreeable to the taste, always preserves the muddy 

 appearance which we have already mentioned. In order to give 

 transparency, recourse has been had to methods which still 

 more diminish the qualities of it : in fact, we kno\\' that fish 

 glue, or even albumen, men' by their mixture with this liquor 

 clarify it very speedilv ; but what all the world perhaps has not 

 observed, is thi-;, that, independently of the bodies suspended 

 in coffee, and which darken the appearanceof it, the gelatine 

 or the albumen envelops the little oil, which floating upon 

 the infusion or decoction of coffee, gives it a slight smell, or 

 rather it is taken up by the depurating matter. The filtra- 

 tion of coffee would ceitainly deserve the preference, if the 

 infusion, by pouring boiling water upon coffee supported 

 on a woollen strainer, did not answer belter than the decoc- 

 tion of coffee, which ought to be rejected. 



There is one obser-'ation as important as the former, which 

 occurs in the preparation of coffee by decoction, and applies 

 against this bad"practiee : it is also grounded upon the pro- 

 perty generally possessed by the extractive matter of vegfeia- 

 bles, of decomposinji the air of the atmosphere and com- 

 bining with oxygen, with which it for'fis an insoluble com- 

 pound, which is separated, aid precipitated to the bottom of 

 the li(|uor: it is at first irianifcsted in the form of very 

 slender pellicles, which become thicker and thicker; it is 

 alsr) to this phaenomenon, nr chemical ron;bination, that we 

 ought to refer the very perceptible discoloratio'i which the 

 decoction of coffee undergoes when the ebullition i^ loo 



lonsr 



