210 METALLIC STAINS 



They are then put for twenty-four hours, or at least over- 

 night, into the gold-bath, which is preferably 1 per cent, gold 

 chloride (see § 400), but may be weaker, down to 0-1 per cent., 

 after which they are just rinsed with water or superficially dried 

 with blotting-paper. The slides are then set up on end in a 

 sloping position, the sections looking downwards, so that pre- 

 cipitates may not fall on them, in glass tubes filled with 1 per 

 cent, formic acid. The tubes are then exposed to light until the 

 gold is reduced, as directed in § 407 sub fin. 



Lee found it advantageous to reduce in weak solution oi formal- 

 dehyde, either with or without formic acid. 



SzuTZ {Zeit. wiss. Mik., xxix., 1912, p. 292) reduces as Apathy 

 for one day, then rinses and puts back for the night into the gold, 

 then for the next day again into the formic acid. 



411. Impregnation of Marine Animals. For some reason the 

 tissues of marine animals do not readily impregnate with gold 

 in the fresh state. It is said by Fol that impregnation succeeds 

 better with spirit specimens. 



412. Preservation of Impregnated Preparations. Preparations 

 may be mounted either in balsam or in acidulated glycerin (1 per 

 cent, formic acid). 



Theoretically they ought to be permanent if the reduction of 

 the metal has been completely effected, but they are very liable 

 to go wrong through after-blackening. Ranvier states that this 

 can be avoided by putting them for a few days into alcohol, 

 which he says possesses the property of stopping the reduction of 

 the gold. 



Blackened preparations may be bleached with cyanide or ferri- 

 cyanide of potassium. Redding employs a weak solution of 

 ferricyanide, Cybulsky a 0-5 per cent, solution of cyanide. 



Preparations may be double-stained with the usual stains 

 (safranin being very much to be recommended), but nuclei will 

 only take the second stain in the case of negative impregnation. 



OTHER METALLIC STAINS 



413. Osmic Acid and Pyrogallol. This method was first pub- 

 lished by Lee in 1887 {La Cellule, iv, p. 110). It consists in 

 putting tissues that have been treated with osmic acid into a 

 weak solution of pyrogallol, in which they quickly turn greenish- 

 black, sometimes too much so. 



Hermann {Arch. mik. Anat., xxxvii, 4, 1891, p. 570) put 

 platino-aceto-osmic material hardened in alcohol for twelve to 

 eighteen hours into raw pyroligneous acid. This acid ought 

 {Ergebnisse der Anat., ii, 1893, p. 28) to be as raw as possible, and 

 to be of a dark brown colour and evil-smelling. (The stain obtained 

 in this wav is not due to a mere reduction of the osmic acid, but 



