J. W. GROVES ON SECTIONS OF ANIMAL TISSUES. 233 



of failure in procuring good stains, to say nothing of their also 

 being the cause of want of success in cutting thin sections. 



These facts are, 1st, that it is necessary to be careful to employ 

 that strength of the hardening or softening fluid which is most suit- 

 able for the tissues to be acted upon. What these strengths should 

 be it would be impossible to tell for every case, but I will give those 

 which are most usually employed, and for further details must refer 

 you to that useful work, "Practical Histology," by Mr. E. A. Schafer. 



The second fact is, that small pieces of material and a large bulk 

 of the fluid must always be employed ; say, half a pint of fluid for 

 pieces equal to one cubic inch. 



The third fact is, that the fluid (even though that may be 

 alcohol), should be changed frequently. How frequently this change 

 should be made, can only be learned by experience, but for all aque- 

 ous solutions it is well to make the first change after a few hours, 

 then every day or two. 



Of the hardening agents chromic acid comes first. This should 

 be used in solutions of various strengths, but provided they are fre- 

 quently changed it is better to err on the side of their being too weak 

 than too strong, for in the latter case, the tissues become friable, and 

 crumble away before the knife or razor ; besides, it is utterly im- 

 possible to stain them properly. 



The most useful strengths are -|- or \ per. cent, aqueous solutions, 

 but before immersing the material in these, it is well to place it in a 

 2 per cent, solution of bichromate of potash or bichromate of am- 

 monia, for some eight or twelve hours, as they have a greater pene- 

 trative power than the simple chromic acid solution, and thus cause 

 the masses to be more evenly hardened. It should be remembered 

 that chromic acid solutions have a tendency to bleach tissues already 

 stained or injected with carmine solutions. 



Bichromate of potash and bichromate of ammonia. Either of 

 these may be used in a 2 per cent, solution ; the latter being the 

 more generally useful.* 



Alcohol. This is perhaps the most generally useful fluid for 

 those who have not much time to devote to the subject, as with it 

 there is less chance of the material spoiling ; moreover, tissues 

 hardened by its means, give the best results with most of the stain- 



* Material hardened by immersion in solutions of chromic acid, bichro- 

 mate of potash, or bichromate of ammonia, should be removed to alcohol at 

 the end of about ten days, or it will become brittle. 



