300 Mr Gardiner, On the general occurrence [May 28, 



Thus as far as regards gallotannic acid its relations and struc- 

 tural formula have at least been stated, but with regard to the 

 other forms of tannic acid we have at present no details. 



Among the many reagents hitherto employed in botanical 

 microchemistry as tests for tannin, none can be regarded as 

 entirely satisfactory. Leaving out of the question such reagents 

 as the soluble salts of barium, mercury, and antimony, whose 

 precipitates in consequence of their white colour can only be 

 recognized with great difficulty, if at all, there remains for con- 

 sideration Schultz's solution (Chlor- Zinc- Iod-), Potassium bi- 

 chromate, and the well known solution of Ferrous sulphate which 

 has been allowed to oxidyze slightly, in order that some Ferric 

 salt may be present. As to Chlor- Zinc- Iod-, which when diluted 

 gives a violet colour with tannin, it must be regarded as highly 

 unsatisfactory, not only because it is a very powerful reagent, but 

 especially because it colours the cell wall and the starch grains, 

 and from the latter reaction is liable to give very delusive results 

 and to suggest some definite connection between tannin and starch, 

 which is certainly not proved to exist from such a reaction and 

 which considered from a chemical point of view is extremely im- 

 probable. Moreover Chlor- Zinc- Iod- gives no reactions with 

 bodies very closely allied to starch, such as glucose. 



Potassic bichromate 1 or chromic acid is an extremely useful 

 reagent producing a well defined gelatinous precipitate of a 

 reddish-yellow colour in the cells containing tannin. But with 

 the nature of this precipitate we are by no means acquainted 

 and from its colour it may as well be the reduced chromic oxide 

 as a doubtful tannate of chromium ; moreover knowing as we 

 do how extremely easily chromic acid is reduced by such bodies 

 as aldehydes, and even by alcohol, there is great probability that 

 many of the precipitations produced by potassic bichromate or 

 chromic acid do not point to tannin at all, but to some other 

 reducing agent present in the cell sap. Another objection is 

 the colour of the salt which of itself stains the tissues. It must 

 however be frankly admitted that in the majority of instances 

 chromic acid appears to be a perfectly satisfactory reagent and is 

 nearly always confirmed by iron. 



For the tannins which give a blueblack precipitate with the 

 mixture of iron sulphates 2 , this reagent is in every way trust- 

 worthy, in that here we have a distinct precipitation or coloura- 

 tion, the character of which admits of no doubt, but with regard to 

 those tannins which give a green precipitate it may, as in the 

 case of the Chromium, be reasonably doubted whether the pre- 



1 Sanio, Bot. Zeit. 3. 18G3. 



- See Sachs, "Einige neue Reactionsmethoden." Site. d. L Akad. in Wten, 

 sxxvi. 18-59. 



