36 BULLETIlSr 448, U. S. DEPARTMENT OP AGEICULTURE. 



make dyeings to any convenient standard depth, and descriptions can 

 only indicate in a general way what may be expected. Not only will 

 the appearance of the dyed wool under the influence of different 

 reagents vary somewhat with the concentration of the dye present, 

 but the shade of the dry fiber also may vary with the concentration of 

 the dye. For example, dyeings from some of the oranges are almost 

 yellow when only a little color is present, but are a much redder shade 

 when more dye is used. 



Color changes similar to those taking place on dyed fibers are pro- 

 duced in most cases by the given reagents in solutions of the dyes, and 

 the conditions are under much more exact control. So in some cases 

 it is advantageous to compare the solution of the dye under examina- 

 tion with solutions of known colors, all being brought as nearly as 

 possible to the same dye concentration, and to the same acid or alkafi 

 normality. An exact statement of shade can be given best by spectre- 

 photometric data (according to Vierort's method) under prescribed 

 conditions of temperature and concentration. This is desirable in 

 some cases, as when dealing with mixtures of Ponceau 2 R and Pon- 

 ceau 3 R. For the somewhat related and more rapid spectroscopic 

 method of measuring the spectral position of maximum light absorp- 

 tion in dilute solutions, the treatises of Formanek, of Formanek and 

 Grandmougin, and of MuUiken may be consulted. However, the 

 "spot reactions," if not of the greatest exactness, are sufficiently exact 

 for most purposes, and are especially convenient in inspection work, 

 where it is well to keep a specimen of the color dyed on cloth. 



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