206 METALLIC STAINS (IMPREGNATION METHODS). 



developing solution, such as is used in photography, or in a warm 

 concentrated solution of tartaric acid, at the temperature of an 

 incubating stove. 



I have myself used the double chloride of gold and sodium with 

 good results. 



CIACCIO (Journ. de Microgr., vii, 1883, p. 38) prefers the double 

 chloride of gold and cadmium. 



FLECHSIG (Die Leitungsbahnen in Gehirn, 1876 ; Arch. Anat. u. 

 Phys., 1884, p. 453) reduces in a 10 per cent, solution of caustic soda. 



NESTEKOFFSKY treats impregnated preparations with a drop of 

 ammonium sulphide, and finishes the reduction in glycerin (quoted 

 from GIERKE'S Fdrberei z. mik. Zivecken). 



BOHM reduces in Pritchard's solution amyl alcohol, 1 ; formic 

 acid, 1 ; water, 98. 



MANFREDI (Arch, per le Sci. med., v, No. 15) puts fresh tissues into 

 gold chloride, 1 per cent., for half an hour ; then oxalic acid, 0-5 per 

 cent., in which they are warmed in a water-bath to 36. Mount in 

 glycerin. Sunny weather is necessary. 



BOCCARDI (Lavori Instit. Fisiol. Napoli, 1886, i, p. 27 ; Journ. 

 Roy. Mic. Soc., 1888, p. 155) recommends oxalic acid of 0-1 per cent, 

 or of 0-25 to 0-3 per cent., or a mixture of 5 c.c. pure formic acid, 

 1 c.c. of 1 per cent, oxalic acid, and 25 c.c. of water, reducing in the 

 dark not longer than two to four hours. 



KOLOSSOW (Zeit. wiss. Mik., v, 1888, p. 52) impregnates for two 

 or three hours in a 1 per cent, solution of gold chloride acidulated 

 with 1 per cent, of HC1, and reduces for two or three days in the dark 

 in a 0-01 per cent, to 0-02 per cent, solution of chromic acid. 



GEBERG (Intern. Monatsschr., x, 1893, p. 205) states that previous 

 treatment of tissues for twenty-four hours with lime-water (ARN- 

 STEIN'S method) greatly helps the reduction. 



BERNHEIM (Arch. Anat. Phys., Phys. Abth., 1892, Supp., p. 29) 

 adds to L 6 WIT'S dilute formic acid a piece of sulphite of sodium 

 (must be fresh and smell strongly of sulphurous acid). 



Dr. LINDSAY JOHNSON writes to me that besides the " sunning " 

 of the impregnating solution recommended above ( 349), the gold 

 should be carefully acidulated with a neutral acetate or formate, or 

 acetic or formic acid, at least twenty-four hours before using ; and 

 then afterwards the tissue must be washed until no reaction occurs 

 to test-paper. 



APATHY (Mikrotechnik, p. 173 ; Mitth. Zool Stat. Neapel, xii, 

 1897, pp. 718 728) lays stress on the necessity of having the objects 

 thoroughly penetrated by light from all sides during the process of 



