STAIN TECHNIQUE 27 



an acid dye, light 4 to 5. The absence of a bitter taste accounts 

 for the former use of this poisonous dye in coloring starchy food 

 such as vermicelli and macaroni. Pianese includes Martius Yellow 

 with Malachite Green and Acid Fuchsine for the differential stain- 

 ing of cancerous tumors. Other technicians use the triple stain 

 for fungous parasites on plants. There is no need for Martius 

 Yellow in the mixture when the yellow can be traced to either a 

 chemical in the fixative or a natural pigment. Instead of Martius 

 Yellow use Sun Yellow, C. I. No. 620. 



10. Naphthol Yellow S, Acid Yellow is an acid dye, light 3 to 

 4. Only a light color is obtained unless acetic acid and potassium 

 dichromate are present in the stain. It is more fast than Martius 

 Yellow and more soluble in water ; acids cause no precipitates in its 

 solution. Its principal value is for the coloring of foods such as 

 noodles and macaroni. 



Class III. Monoazo Colouring Matters 



Although the azo chromophore is basic in character, the azo 

 dyes may be either basic or acid according to the auxochromes 

 present. The number of chromophores in the dye molecule 

 determines the classification as monoazo, disazo, trisazo, or polyazo 

 dyes. Those azo dyes which are insoluble in water or alcohol may 

 be used as fat soluble stains. Azo dyes differ through all degrees 

 of fastness to light ; many are fugitive whereas others can satisfy 

 the most exacting requirements in this respect. In some instances 

 the addition of potassium dichromate to the stain increases the 

 intensity of the color and its fastness. 



16. Acid Yellow, Fast Yellow S is a direct dye, light 2 to 3; 

 i gram in 95 ml. of distilled water, 1 ml. of 1 per cent sulfuric acid, 

 and 5 ml. of \ per cent potassium dichromate ; filter. Sections of 

 animal or plant materials are stained a clear fast orange-brown 

 that is not very brilliant. 



20. Chrysoidine Y, and 21. Chrysoidine R are basic dyes, 

 light 4; the former dye is more soluble. The stain decomposes 

 slowly on standing. These dyes are similar to Bismarck Brown in 

 color but they are not its equal in fastness. Sharp contrasts are 



