44 A NEW METHOD OF ASSAYING TANNATES. 



The second Nessler bottle, labelled " 2 permille," gets two 

 drops of the -001 tannin solution and is filled with the iron 

 chloride solution like the first. In this way each following 

 Nessler bottle gets one drop more of the "001 solution of tannic 

 ac;d. So many drops, so many per mille, of tannin are put in 

 the bottle. Ten drops of '001 is equal to one drop of the 1 per 

 cent, solution. So we put in the tenth bottle one drop of the 1 

 per cent, solution of tannin, fill it with the iron solution and 

 label it " 10 per mille." In the fifteenth bottle, to be labelled 

 " 15 per mille," we put one drop of the 1 per cent, tannin 

 solution and five drops of the "001 solution; in the twentieth 

 bottle two drops of the 1 per cent, solution ; and in the fifty-fifth 

 bottle, to be labelled " 55 per mille," five drops of the 1 per cent, 

 and five drops of the 1 per mille tannin solution, etc. 



The tanmn, on addition of the iron, forms a black ink, 

 darker or lighter according to the lower or higher percentage of 

 tannin brought into contact with the ferric -chloride. The colour 

 of this ink is not altered for many months, as the mucilage of 

 arabin keeps the small particles of ferric-tannate suspended in 

 the liquid and the small amount of benzoic acid prevents the 

 growth of fungi. 



A very good colorimetric scala, available for a long time, is 

 obtained in this way. 



II. — Process of Assaying. 



Now, to find out the percentage of tannin contained in a 

 body to be examined, we take 10 grammes of the material, put 

 it in 90 grammes of boiling water (or 5 to 45) and keep it boiling 

 for half an hour, adding constantly as much water as is lost by 

 evaporation. Of this decoction or solution we put a part in a 

 patent drop bottle and let one drop out of it fall in an empty 

 Nessler bottle, filling it up with ferric-chloride solution. At once 

 the ferric-tannate is formed, being kept in suspension by the 

 arable gum. By comparing the colour of this test-bottle with 

 the colour of the scala-bottles the percentage of the solution is 

 easily ascertained. The material then contains ten times this 

 amount, having been boiled with 9 parts of water. 



As the gallic acid gives the same reaction as the tannin,, 

 the percentage of it has to be determined for a correct assay ; 

 but no notice need be taken of it as it is generally present in 

 very small quantities. 



