374 NEEVES. 



of the clearness of the images. SEHRWALD (1. c., p. 456) finds 

 that this evil can be avoided by putting the tissues into 

 gelatin solution before bringing them into the silver-bath. A 

 10 per cent, solution of gelatin in water may be made. The 

 tissues are imbedded in this in a paper imbedding box with 

 the aid of a little heat (the gelatin melting at a sufficiently 

 low temperature), and are brought therein into the silver- 

 bath. After the silvering the gelatin is removed by warm 

 water saturated with chromate of silver. 



709. SAMASSA (op. cit., vii, 1, 1890, p. 26) points out that 

 bichromate of silver is not soluble either in absolute alcohol, 

 toluol, xylol, paraffin, or Canada balsam, nor otherwise chemi- 

 cally affected by them ; and that therefore Sehrwald's ex- 

 planation is untenable. He thinks that the true explanation 

 of the deterioration of the preparations is that the precipitate 

 forming the impregnation is little by little floated away from 

 the tissues by the mechanical force of the diffusion currents 

 set up on the passage of the preparations through the dif- 

 ferent reagents, and particularly those long-continued ones 

 caused by the slow drying of balsam under a cover-glass. 

 He recommends, therefore, simply that the preparations be 

 preserved without a cover. 



710. GREPPIN (Arch. f. Anat. u. Entw., Anat. Abth., 1889, 

 Supp., p. 55; Zeit. f. wiss. Mik.,vii, 1, 1890, p. 66) finds that 

 by means of hydrobromic acid, suggested by NEUMANN, pre- 

 parations made by the slow method may be rendered suffi- 

 ciently resistent to bear mounting under a cover. After 

 silvering, sections are made with a freezing microtome and 

 treated for thirty to forty seconds with 10 per cent, solution 

 of hydrobromic acid, and may then be well washed in several 

 changes of water and mounted in the usual way. If they be 

 cleared in clove oil and exposed therein to sunlight for ten or 

 fifteen minutes, they will take on a violet tone, and details 

 will be more strongly brought out. It is sometimes well to 

 treat them, after the 10 per cent, acid, with 40 per cent, acid 

 for twenty to thirty seconds. They may also be treated by 

 Pal's modification of Weigert's haematoxylin process. 



711. OBREGIA ( Virchow's Archiv, cxxii, 1890, p. 387; Zeit. 

 f. wise. Mik., viii, 1, 1891, p. 97; Journ. Roy. Mic. Soc., 1891, 



