186 



Biological Stains 



In the procedure for staining bacteria in soil, as developed by 

 Conn (1918) and by Winogradsky (1924), the matter is still 

 further complicated by the fact that the dye must be of such a 

 color and such a strong acid as to stain the bacteria, but not the 

 dead organic matter present. 



Another factor of much importance in the staining action of 

 these dyes is the amount of mineral salt present. It has, in fact, 

 been shown by Conn and Holmes (1928) that the intensity with 

 which a dye of this group can stain bacteria may be greatly in- 

 creased by adding a minute amount (0.001 to 0.1%) of some 

 mineral salt such as CaCl2. This matter is discussed in Chapter II. 



There has been in the past much mislabeling of the dyes of this 

 group. Thus it is evident that the eosin used by Mallory when he 

 first worked out his eosin-methylene-blue technic must have been 

 a phloxine or some closely related dye rather than a true eosin. 

 There is evidence that erythrosin B and phloxine have been put on 

 the market labeled eosin bluish; so has a blend of eosin Y shaded 

 with some one of the higher members of the group. Phloxine has 

 been sold as magdala red and is even now sometimes marketed 

 under that name both by American and foreign concerns. All 

 this has caused much confusion; but dealers in stains are using 

 greater caution in the matter at present. 



Rl 



FLUORESCEIN 



C. I. NO. 766 



This is the simplest of the fluoran dyes, and is the mother sub- 

 stance of the eosins. Its sodium salt is known as uranin. 



HO 



O 



OH 



NaO 



O 



-0 



O 



_l 

 CO 



COONa 



C20H12O5; Mol Wt. 332.296 

 Fluorescein 



C2oHio05Na2; Mol. Wt. 376.274 

 Uranin 



{An acid dye: absorption maximum about 490) 



Solubility at 26°C: 



Of fluorescein: in water ^ 0.03%; in alcohoU 2.21%. 

 Of uranin: in watery 50.02%q; in alcohol^ 7.19%o. 



