FIXING AND HARDENING AGENTS. 45 



It is not well to take the platinum chloride stronger than 

 here given, as too strong solutions have a tendency to crys- 

 tallise out on the tissues. HENNEGUY, who has worked a 

 great deal with this reagent, and recommends it highly, says 

 (Lerons *ur la Cellule, Paris, Carre, 1896, p. 61) that it is 

 well only to add the acetic or formic acid just before using, 

 as it frequently provokes a spontaneous reduction of the 

 osmium and platinum to such an extent that the mixture be- 

 comes quite black. 



This mixture was imagined for the preliminary hardening 

 of retina, being allowed to act for two hours only, and then 

 being followed by final hardening in pure bichromate solution. 

 But it has proved applicable to other structures, and gives 

 excellent results. The function of the osmic acid in the 

 present formula is to enhance the hardening- energy of the 

 mixture. Dr. Lindsay Johnson writes me that " it greatly 

 reduces the length of time necessary for hardening, three 

 days being the time 'from removal of the organ to its being 

 in celloidiii under dilute spirit." 



This mixture may be used for fixing, in some cases with 

 the best results. HENNEGUY (/. c.) says it contracts the more 

 spongy sorts of protoplasm less than mixture of FLKMMING. 

 I think highly of it for certain objects. 



50. Platino-aceto-osmic Acid (HERMANN'S solution). This 

 valuable reagent is historically a modification of Flemming's 

 solution,, platinum chloride being taken instead of chromic 

 acid. HERMANN (Arch. f. mik. An at., xxxiv, 18S9, p. 58) 

 substitutes 1 per cent, platinic chloride for the chromic acid 

 in Flemming's strong formula for chromo-aceto-osmic acid 

 ( 47), the other ingredients either remaining as before, or 

 the osmium being diminished one half ; thus, 1 per cent, 

 platinic chloride 15 parts, glacial acetic acid 1 part, and 2 

 per cent, osrnic acid either 4 parts or only 2 parts. Hermann 

 found that protoplasmic structures are thus better preserved 

 than Avith the chromic mixture. 



The after-treatment and staining should be the same as 

 for objects treated with Flemming's solution. 



After considerable experience of this reagent I find that 

 it has the advantage of giving more colourless preparations, 

 and it may be, in some cases, a more delicate fixation. But 



