230 ON THE AMOUNT OF TANNINE IN 



ward method of filtration in preference to the upward, as a clear filtrate 

 is more easily obtained, and the sponge does not require such frequent 

 washing ; in the latter case, the pores becoming soon choked up with 

 particles of the precipitate. 



We have noted the colour of the precipitates with sesqui-chloride of 

 iron ; this test has been found by Dr. Stenhouse (Proc. E. Soc. London, 

 No. 46, vol. ii.) to be a pretty accurate indication of the variety of tannic 

 acid which may be present — i.e., whether it belongs to the class of conjugate 

 sugar compounds (glucosides) or not. We had originally intended, and, 

 indeed, commenced a further investigation into the nature of the tannic 

 acid present, but as we considered tins would have no practical advantage, 

 we relinquished this part of the investigation. 



It is a well-known fact that pure tannine cannot be kept unchanged in 

 a state of solution if exposed to the atmosphere ; it attracts oxygen from the 

 air, and is decomposed into gallic and carbonic acids. As the researches of 

 Stenhouse, and other chemists, have rendered it probable that there are 

 several varieties of tannine, it is not unlikely that some of these will be 

 less stable than others, and that even the decomposition of the same 

 tannine may be accelerated or retarded by some principles existing along 

 with it in the particular substance from which it is derived. 



We are not aware that any experiments have been made as to the com- 

 parative rapidity with which the tannine in different substances decom- 

 poses when in a state of solution, and the determination of this point 

 appears to us to be scarcely inferior in point of practical importance to the 

 estimation of the quantity of tannine itself. We have, therefore, deter- 

 mined the total quantity of tannine in the different astringent substances 

 we examined, and also the comparative rapidity with which the tamxine 

 contained in them undergoes decomposition. In order to render the inves- 

 tigation on this latter point as complete as possible, we have determined 

 not only the amount of tannine which decomposes in the infusion of the 

 substances, but we thought it important to ascertain whether the decom- 

 position was influenced in any way by the insoluble substances which 

 remained behind in the exhausted residue. Our experiments, which were 

 necessarily made on the small scale, were conducted in the following 

 manner : — 



We took two portions of several of the solutions, as prepared for analysis, 

 and left them in beakers covered with blotting paper, for a week, exposed 

 to the action of the air, in a room having a mean temperature of about 

 60° Fah. One portion was placed on the bark residue remaining undissolved, 

 the other was left by itself. At the end of a week the solution became 

 slightly ropy ; the amount of alteration in the quantity of tannine will be 

 seen below ; if the temperature was lower than 50° Fah., the liquid became 

 much more ropy and mildewed, and in a much shorter time to such an 

 extent, indeed, that we were unable to precipitate the tannine. The 

 annexed experiments show that the difference in the amount of tannine 



