48 CHAPTEE IV. 



1895, p. 61) expressly referred to the fixing liquid. In any 

 case the end is more simply attained by taking simply acid 

 alcohol, as recommended by me so long ago as 1880 (Mitth. 

 Z. Stat. Neapel, ii, p. 7), which may be taken weaker or 

 stronger according to the nature of the objects." 



53. Chromic Acid and Platinic Chloride (MEKKEL'S Macula 

 lutea des Menschen,, Leipzig, 1870, p. 19). Equal volumes of 

 1'400 solution of chromic acid and 1*400 solution of platinic 

 chloi'ide (PtClJ. Objects should remain in it for several 

 hours or even days, as it does not harden very rapidly. 

 After washing out with alcohol of 50 per cent, to 70 per 

 cent., objects stain excellently, notwithstanding the ad- 

 mixture of chromic acid. If objects that have been fixed 

 by osmic acid be put into it for some hours, blackening is 

 said to be effectually prevented. 



This is an excellent hardening medium for delicate 

 objects. Merkel states that he allowed from three to four 

 days for the action of the fluid for the retina ; for Annelids 

 Eisig employs an immersion of three to five hours, and 

 transfers to 70 per cent, alcohol ; for small leeches Whit- 

 man finds one hour sufficient, and transfers to 50 per cent, 

 alcohol. 



Whitman recommends, for the hardening of pelagic fish 

 ova, a stronger mixture (due, I believe to Eisig), viz. 



0'25 per cent, solution of platinum chloride . 1 vol. 



1 per cent, solution of chromic acid . . 1 ,, 



The ova to remain in it one or two days (WHITMAN, Methods 

 in Micro. Anat., p. 153j. 



Salts. 



54. Chromates. The chromates are amongst the oldest and 

 best tried of hardening agents. The bichromate of potash 

 especially was at one time universally employed for harden- 

 ing all sorts of tissues, and a great amount of classical 

 work has been done with it. 



About eighteen years ago, however, bichromate fell into 

 disrepute in consequence of a criticism on its action made 

 by FLEMMING. I-YKMMING pointed out (Arch. f. inik. Anat., 

 xviii, 1880, p. 352) that though it preserves cytoplasm well 



