MS 31.0 



METAL STAINS 



549 



tained at 55°C. The stain depends for its 

 effectiveness, first, on the inhibition of the 

 staining of nervous elements by exposure 

 to uranium nitrate, and second, on the 

 selective deposition of the silver from a 

 colloidal environment which is, in this 

 case, produced with gum mastic. 



All four solutions should be prepared 

 before the sections are started. The first 

 solution is 1% uranium (uranyl) nitrate 

 in 70% alcohol. Both the metaUic salt 

 and the alcohol itself should be chemically 

 pure, should be kept in chemically clean 

 bottles, and used in chemically clean 

 dishes. The next solution is a 10% solu- 

 tion of gum mastic in absolute alcohol. 

 This is best prepared by selecting clear, 

 transparent, hght-colored pieces of gum 

 mastic from a large quantity of the gum, 

 grinding the selected pieces of gum mastic 

 of the required weight with dry sand in a 

 mortar, then flooding tliis mixture with 

 absolute alcohol, and shaking until solu- 

 tion is complete. The sand is then re- 

 moved by filtration through glass wool or 

 some other relatively coarse material. 

 The tliird requirement is a 1% solution 

 of silver nitrate in triple-distilled water 

 and must again, of course, be maintained 

 in chemically clean bottles. The fourth 

 solution, which is the developer, is very 

 complicated, but is necessary in the 

 peculiar circumstances under which this 

 technique is employed. The formula will 

 be found under AMS 21.1 Dieterle 1927; 

 but it is so difficult to make up that the 

 brief description there given must be aug- 

 mented. In the first place, mix 150 milli- 

 liters of triple-distilled water with 30 of 

 reagent grade acetone. The technique will 

 be seriously impaired if a low quaUty of 

 acetone is employed. In this mixture, dis- 

 solve first 43»^ grams of hydroquinone, and 

 after the solution is complete, a 9-4-gram 

 of anhydrous sodium sulfite. When the 

 solution of sodium sulfite is complete, add 

 first 30 milliliters of pyridine, and second 

 30 milhliters of 40 % neutraUzed formalde- 

 hyde. This forms, when it has been 

 brought by triple-distilled water to a total 

 of 270 milhliters, a relatively stable solu- 

 tion. To this then add, drop by drop, ten 

 millihters of a 10 % solution of gum mastic. 

 This emulsion is stable for some days, and 



can usually be re-emulsified by shaking, 

 should it tend to separate. 



When these solutions have been pre- 

 pared, and an adequate supply of chem- 

 ically clean coplin jars provided, the sec- 

 tions are immersed for 30 minutes at 55°C. 

 in the uranium nitrate solution. They are 

 then washed in at least three changes of 

 triple-distilled water before being placed 

 in 90% alcohol for two minutes. When 

 they are sufficiently dehydrated they are 

 flooded with 10% gum mastic, or placed 

 in a jar of 10% gum mastic for about 30 

 seconds. It is less essential to impregnate 

 each individual section than to insure that 

 there is a sohd film of the solution over 

 the surface of the slide. Each slide is then 

 removed individually, drained by one 

 corner to remove surplus gum mastic, and 

 the back of the shde hghtly wiped. They 

 are then rinsed very, very briefly in 90% 

 alcohol to remove the remains of the gum 

 mastic from the back of the shde, and to 

 make sure that drops of gum mastic do 

 not remain between the sections. Immedi- 

 ately after this brief rinse, they are 

 dropped into triple-distilled water where 

 the gum mastic is, of course, precipitated 

 in a colloidal form over the surface and 

 through the material of each section. Each 

 shde is best handled individually in its 

 successive changes through alcohol, mas- 

 tic, and water, all the slides then being 

 accumulated together in triple-distilled 

 water. 



The utmost care must be taken to pro- 

 vide a chemically clean coplin jar in wliich 

 to place the silver nitrate solution which 

 is next used. This is preheated to 55°C., 

 preferably by leaving it for some hours in 

 the embedding oven. It is then removed 

 briefly from the oven, the slides are 

 dropped into it, and it is returned to the 

 oven for from one to six hours. It is essen- 

 tial that it be kept in the dark during this 

 period. After removal from silver nitrate 

 the sections should be a very pale yellow 

 color. If they appear blackened, it is usu- 

 ally from some impurity in the gum 

 mastic, and there is nothing to do save to 

 take a fresh sample of gum and to experi- 

 ment with it. 



After removal from silver nitrate, the 

 sections are washed in at least three 



