THE WOODS OF THE UNITED STATES. 



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wood may be determined by microscopic examination of its structure ; that is, two specimens of the wood of any 

 species to which the census tests have been applied being given, their relative values can be determined by an 

 examination of their structure as well as or better than by any elaborate experiments. 



TANNIN VALUES. 



The amount of tannin contained in the bark of various trees of the United States has been determined. 



These determinations give the proportion of tannin. They do not indicate the real value of the bark of the 

 species for tanning, which can only be obtained by actual experiments made on a large scale, other properties in 

 the bark, beside the percentage of tannin, affecting the value of the leather prepared with it. 



These determinations must therefore be regarded as approximations, which will serve, in some cases, to 

 indicate species not now in general use for this purpose, which may be looked to as possible sources of tannin 

 supply. 



The methods adopted by Mr. Sharpies in making these determinations are described by him as follows : 



The tanuiu in each case was determined in the rossed bark; that is, bark deprived of the main part of the outside coating. The 

 method employed was that devised by Lowenthal, which may be thus briefly described : A standard decoction of the bark is titrated 

 with permanganate of potash, a quantity of indigo being first added to it. In a second portion the tannin is precipitated by means of 

 gelatine, and the gallic acid in the liquid again determined by permanganate and indigo. The difference between these two readings 

 gives the amount of tannin in the bark, the value of the permanganate having previously been determined by pure tannic acid, or by 

 oxalic acid and calculation. 



The bark of the following species has been examined : 



