list previously published. The Schultz number of each of them is 

 given for reference purposes, however, in the list in the appendix, 

 p. 106. 



DYE SOLUBILITIES 



Textile dyes are never of a high degree of purity. Some of the 

 impurities are accidental; others are added intentionally so that 

 dyers can obtain the desired shade without having to measure out 

 dyes in very small quantities. Inasmuch as the early biological 

 stains were textile dyes without much, if any, modification, it is 

 natural that some of them should also have been of low dye con- 

 tent, and also that different batches should have been of various 

 degrees of purity. In general the post-war dyes are much more 

 pure thail those available before the war. This makes it difficult 

 to prepare stain solutions identical in strength with those prepared 

 before the war. 



There are two general types of stain formulae: in one a definite 

 weight of dry dye is specified; in the other a certain volume of a 

 saturated (generally alcoholic) solution of the dye. Each type of 

 formula has its own possibilities of error; and to appreciate the 

 problem it is necessary to understand certain facts in regard to the 

 solubilities of dves. 



The error inherent in the first type of formula is plain at a 

 glance. If two different staining solutions are made up containing 

 1 g. per 100 cc. of dry methylene blue, and in one case the actual 

 dye content of the dry stain is 90 percent, while in the other only 

 55 percent (a difference actually observed in samples on the mar- 

 ket), it is plain that the two solutions must differ greatly in their 

 strength. For this reason an early recommendation of the Com- 

 mission (1923b) was that formulae of the second type be preferred, 

 on the assumption that a saturated solution of a dye would be more 

 likely to be of constant dye content than different lots of dry stain 

 bought in the market. 



This recommendation, however, was made without complete 

 understanding of the actual facts of the case. The amount of a dye 

 that will go into solution in either water or alcohol depends upon 

 the amount of mineral salts present. If a dye contains a large per- 

 centage of sodium chloride, for instance, a saturated solution will 

 be of considerably lower actual dye content than if the dye were 

 free or nearly free from salt; the sodium chloride prevents the 

 solvent from taking up as much of the dye as it would normally. 

 For this reason two staining solutions each containing 10 percent 

 by volume of a saturated solution of the two methylene blues above 

 mentioned would be quite different from each other in actual dye 

 content, altho possibly more nearly alike than if they had been pre- 

 pared with identical weights of the dry stain. 



As soon as these facts were fully understood, the Commission 

 (1923e) modified its recommendation. It is plain that the only way 



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