444 On the Comhination of Acids 



2. That galliuits evidently contain abundance of gallic 

 acid, and that, this acid having a strong affinity for tannin, 

 the two bodies should necesarily neutralize each other in 

 part. 



3. Because there are many remarkable instances of the 

 power of some vegetable substances to neutralize acids. 



4. Lastly, that tannin precipitated from a decoction of 

 gallnuts by an alkali or carbonate of ammonia, contains, 

 according to my experiments, a good deal of gallic acid aS 

 well as a small portion of saline base. The proof I have of 

 this is, that it produces with almost all the metallic solu- 

 tions the same effects as gallic acid, or gallate of potash. 

 Thus it gives red precipitates with solutions of the per- 

 oxides of mercury, blackish gray with solutions of per- 

 oxides of iron, a blue one to those less oxidized, the colour 

 of wine lees (lie de vin) to those least oxidized. It follows 

 then that we are not yet acquainted with the pure tannin of 

 gallnuts, and of most other substances, and consequently 

 that we know not how to act with them on metallic solu- 

 tions, &c. Inqunies which have this object in view can- 

 r.ot but be interesting; and I propose to pursue them, in 

 as much as they will on some future day allow me to bring 

 forward the question I am now treating of. 



If, after having examined the vegetable substances which 

 readily combine with acids, a search is made among ani- 

 mal substances endowed with the same properties, five will 

 evidently occur: curdy matter, albumen, picromi:!,, gelatin, 

 and urea. This properly is generally found in the curdy 

 matter, as it is known that acids coagulate milk, that the 

 coagulum contains an acid, ar\d that this acid is even sen- 

 sible to turiisi)lQ paper. Of course the affinity belwecn 

 those two bodies is well marked ; yet it is not sufficiently 

 strong to destroy all doubts to the contrary ; for I am sa- 

 tisfied that the excess, and perha|-)S also the whole, of the 

 acid may be abstracted by means of a large quantity of 

 water. 



If water alone be sufficient to take the acid from the 

 curdv matter, it fads to produce the same eftecc when the 

 acid is united to albumen ; for the washings may be con- 

 tinued to at' unlimited extent, and there will be aKvays 

 found acid undissolved. 



We must conclude tiierefore, that ibis animal substance 

 exerts on acids a more powerful attraction than the curdy 

 matter; but this action varies in itself according to the 

 concentrated state of the acid. As the acid is saturated 

 with water^ the results are combinations more or less in- 



soUibl?, 



