50 HARDENING AGENTS. 



78. Bichromate and Platinic Mixture (LINDSAY JOHNSON'S 

 Mixture; communicated by Dr. Lindsay Johnson). 



Bichromate of potash (2'5 per cent.) . 65 parts. 

 Osmic acid (2 per cent.) . . 15 



Platinic chloride (2 per cent.) . 15 



Acetic or formic acid . . . 5 



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 I have no doubt that it will prove applicable to other 

 structures, and give excellent results as a slow and gentle 

 hardening agent. For this purpose I would suggest that the 

 osmium might be diminished or omitted, the proportion of 

 Pt. C1 4 increased, and that of acetic acid diminished. 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 celloidin under dilute 

 spirit." 



" If the osmium has a tendency to blacken, this may be 

 entirely prevented and a beautiful delicate chestnut-brown 

 deepening towards Bartholozzi-red tint obtained by adding 

 ten parts of 5 per cent, solution of nitrate of uranium, 

 which forms a layer of uranium on the top of the reduced 

 platinum and osmium (one or both)." 



79. Bichromates and Alcohol. Mixtures of either bichro- 

 mate with alcohol may be employed, and have a more rapid 

 action than the aqueous solution. Thus HAMILTON takes for 

 hardening brain a mixture of 1 part methylated spirits with 

 3 parts of solution of Miiller (see the chapter on the Central 

 Nervous System in Part II. ; see also KULTSCHIZKY'S Mixture, 

 ante, 43). Preparations should be kept in the dark during 

 the process of hardening in these mixtures. 



80. Bichromate of Ammonia. A review of the literature of the sub- 

 ject shows that this salt is in considerable favour, for what precise motive is 

 not apparent. Its action is very similar to that of the potassium salt. Fol 

 says that it penetrates somewhat more rapidly, and hardens somewhat more 

 slowly. It should be employed in somewhat stronger solutions, up to 5 per 

 cent. 



81. Neutral Chromate of Ammonia is preferred by some anatomists. 





