General Nature of Dyes 39 



correction he has to make if he has a dye sample of different dye 

 content. Many individual writers, however, especially writers of 

 text books, have not followed this system. There are various 

 reasons such a plan has not been generally adopted. The chief 

 reason is that there is much variation in the dye content of in- 

 dividual batches, and it is obvious that the staining formulae can- 

 not be generally put in the form recommended by the Commission 

 unless the manufacturers print the actual dye content of the batch 

 of stain on the container in which it is sold. Until about 1930 this 

 was never done. Since about that time, however, the Stain Com- 

 mission has been issuing its certification only when the dye con- 

 tent is printed on the label, except in instances where the neces- 

 sary analytical methods are not available. This policy has grad- 

 ually been changing the situation. At the present time almost all 

 stains on the American market, except natural dyes, compound 

 dyes like the blood stains, and a few complex dyes for which good 

 analytical methods are still unavailable, are labeled as to their dye 

 content. In all Commission publications (including Stain Tech- 

 nology) all formulae are given in terms of grams of Commission 

 certified stains (unless the dye in question has never been certi- 

 fied), and as certification is now issued only on samples that vary 

 within narrow limits, this amounts to recommending definite 

 weights of actual dye. Gradually authors of papers published 

 elsewhere are getting into the same habit; so the situation is much 

 more satisfactory than a few decades ago. 



In the present edition of this book there are listed, both under 

 the individual dyes and in Table 6 in the Appendix, the solubilities 

 of pure dyes in 95% alcohol and in water at 20° of the most com- 

 monly used stains. These data were originally published by 

 Holmes (1927, 1928, 1929) and are based on determinations made 

 by a cooperating laboratory of the U. S. Department of Agricul- 

 ture. The author of these papers experienced considerable diffi- 

 culty in obtaining the information given therein because of the 

 strong effect of impurities upon the amount of dye capable of going 

 into solution. The commercial samples of the dyes are rarely pure, 

 and Holmes found it necessary to recrystallize each sample before 

 determining solubility. This fact, therefore, must be taken into 

 account in interpreting the figures. To obtain a solution of many 

 dyes as strong as the figure given in the table, the dye sample em- 

 ployed must be strictly pure. As this is practically never true, 

 the user of stains can rarely expect to make a saturated solution of 

 the theoretically possible strength. These data nevertheless are 

 useful in indicating the amount of stain to employ in order to 

 obtain a saturated solution. 



The data given in Table 7 were compiled in the hope of furnish- 

 ing information of more practical value to users of stains than those 

 published by Holmes; these were obtained through the courtesy 



