40 CHAPTEE IV. 



specimens, and found it admirable for these purposes. B Lit preparations 

 made to test its value from a cytoloyical point of view have given me 

 only second-rate results. It is now little used. 



49, Chromic Acid and Platinum Chloride (Meekbl's Macula 

 lidea des Menschen, Leipzig, 1870, p. 19). — -Equal volumes of 

 I'-lOO solutiion of chromic acid aud r400 solution of platinum 

 chloride. Objects should remain in it for several hours or 

 even days. After washing out with alcohol of 50 per cent, 

 to 70 per cent., objects stain excellently. 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 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 Whitman finds one hour sufficient, and 

 transfers to 50 per cent, alcohol. 



A similar mixture, with the addition of 0'25 to O'l per 

 cent, of acetic acid, is recommended by Brass for Protozoa ; 

 and Lavdowsky" has used for nuclei a mixture of 10 parts of 

 1 per cent, chromic acid, 5 of 1 per cent, platinum chloride, 

 and 100 of 5 per cent, acetic acid. 



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. 153). 



Salts. 



50. 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 hardening 

 all sorts of tissues. 



Flemming [Arch. mil-. Anat., xviii, 1880, p. 352) pointed 

 out that though it preserves cytoplasm well it causes 

 chromatin to swell, and therefore should not be employed 

 for the study of nuclei. But, duly corrected with acetic acid, 

 it affords a correct and fine fixation of nuclei ; whilst pre- 



