ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1059 



ductory remarks close with a reference to the so-called mordants, the effect 

 and use of which is well known. The author believes that for microscopical 

 research such aids are of little or no value for staining purposes, as much 

 of what is afterwards seen and described in the preparation must be ascribed 

 to structural alterations due to the action of the mordants. 



After enumerating the various anilin dyes, the classification of which is 

 adopted from Hummel,* the author proceeds to discuss the characters and 

 staining properties of Congo red and benzo-purpurin. Congo red is soluble 

 in water, the solution being bromide-red. The specific gravity of the com- 

 mercial article is 2 • 2149. The reaction of the chemically pure preparation 

 is neutral, that of the trade preparation alkaline. To obtain this pure article 

 the dye is dissolved in about 20 parts water, and is then precipitated by the 

 aid of heat with an equal volume of saturated salt solution. After cooling, 

 the dye is washed off the filter with the salt solution. The least quantity 

 of a free acid turns the Congo solution blue, hence Congo is a delicate test 

 for a free acid. This double action has been turned to account for demon- 

 strating the presence of free acids in certain animals, and the alkaline 

 reaction of the living tissue in others. According to the author, the watery 

 solution of Congo is alone suitable for microscopical purposes, for, although 

 miscible with glycerin and turpentine oil, the results therefrom are not 

 satisfactory. For tissue staining, a concentrated watery solution stains 

 both fresh and preserved material. Blood-corpuscles must be dried at 80° 

 for twelve hours before using the watery solution, otherwise the action of 

 the dye quite destroys the tissue. With regard to the staining of animal 

 tissue generally, it appears that the plasma takes up the stain more freely 

 than the nuclei, which are frequently devoid of colour. The hue varies 

 from yellow to red, and preserved material stains better than fresh. One 

 interesting example of its action is that of a section of a fibroneuroma, in 

 which the connective tissue became of a dark orange, and the nervous tissue 

 received a bright-orange stain. Transverse sections of nerves only stained 

 in the sheaths, the axis-cylinder being unaffected. 



Benzo-purpurin is obtainable in two shades, 1 B and 4 B. It is soluble 

 in water, and has approximately the same hue as Congo, but is not affected 

 by acids in the same way as Congo. Its reaction is neutral. Cover-glass 

 preparations dried for ten hours at 200° are said to be successful. In general 

 the stain is somewhat similar to that of Congo, but as a rule the hue is 

 redder. 



Rosanilin and Pararosanilin.t — Dr. P. G. Unna has tried to solve the 

 question whether the appearance of lepra bacillus as threads containing 

 cocci is dependent on the Lutz procedure, a combination of Gram's method 

 with decoloration in nitric acid ; whether this special appearance is due to a 

 reaction between the gentian-violet and iodine, and how this peculiarity 

 can be explained. 



After numerous experiments with various chemically pure dyes the author 

 discovered that only the pararosanilins, to which gentian-violet belongs, 

 possess the property (when used as stated) of showing lepra bacilli as 

 " coccothrix," while rosanilin, under similar circumstances, presented the 

 same micro-organisms as bacilli. This difference is so constant that by 

 their aid it is always possible under the Microscope to distinguish the two 

 dyes, and this is all the more striking, as between rosanilin and pararosa- 

 nilin there is only a slight chemical difference, CH3 replacing H. 



The author, furthermore, showed the relation of the iodine preparation 



' ' The Dyeing of Textile Fabrics,' London, 1885, 

 t Dermatol. Studjen, 1887, Heft iv., 73 pp. 



