258 CHAPTER XVIII. 



formic acid. A few drops of formol added to the tube with 

 the objects will suffice. 



359, Impregnation of Marine Animals. For some reason 

 that I am unable to explain, 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. 



360. Ulterior Treatment of Impregnated Preparations. Pre- 

 parations 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 the preparations for a 

 few days into alcohol, which he says possesses the property 

 of stopping the reduction of the gold. But this must be 

 taken to mean that by this device the period of usefulness 

 of the preparations may be prolonged for some time, not 

 indefinitely. 



Blackened preparations may be bleached with cyanide or 

 ferrocyanide of potassium. REDDING employs a weak solution 

 of ferrocyanide ; CYBULSKY a 0*5 per cent, solution of cyanide. 

 But the results are far from being perfectly satisfactory. 



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 im- 

 pregnation. 



Other Metallic St 



ains. 



361. Osmic Acid and Pyrogallol. This method was first 

 published by me in 1887 (La Cellule, t. iv, fasc. 1, p. 110). 

 It consists essentially in putting tissues that have been 

 treated with osmic acid into a weak solution of pyrogallol, 

 in which they quickly turn greenish black, sometimes much 

 too much so. 



At that time I had only tried the reaction with tissues 

 impregnated with pure osmic acid. Since then HERMANN 

 (Arch. f. mik. Anat.y xxxvii, 4, 1891, p. 570) tried it with 



