standing in presence of the air; with lead acetate it shall yield first a colorless and 

 then a blue precipitate darkened by the air. 



2. When used by one of the standard methods such as that of Haidenhain for 

 staining actively growing root tips or other good cytological material showing 

 mitosis, it shall give a sharp, vigorous black staining of the chromosomes. When 

 used in the Delafield method, it must give a clear-cut blue picture of chromatic 

 material. These tests shall be made by someone skilled in the technic involved. 



3. Solutions shall show no discoloration and shall retain their staining qualities 

 upon standing for a period of three or four weeks. 



SPECIFICATIONS FOR THIONIN 



1. Samples of thionin (Syn. Lauth's violet) must correspond to Colour Index 

 No. 920 and be characterized by an absorption curve Avith a maximum at about 

 aOififi as determined in a ,001 per cent solution in a layer 1 cc. thick in a spectro- 

 photometer. 



2. Samples submitted for certification must have a dye content of at least 

 85 per cent as determined by titanous chloride reduction in an atmosphere of 

 carbon dioxide. 



3. The samples should prove satisfactory for staining frozen sections of fresh 

 animal tissue and should show good metachromatic effects when applied in a 1 

 per cent solution for 1 to o minutes, followed by rapid washing and mounting in 

 water. Their performance in this matter shall be judged by someone familiar with 

 the technic. 



4. The samples should prove satisfactory for staining bacteria by the "little 

 plate" technic described by W. D. Frost in Jr. Inf. Dis. 19, (1916) p. 273-287 



137 



