16 FIXING. 



so that they cannot be easily removed from them by washing. 

 This is a consideration of great importance in view of ulterior 

 operations, and most particularly in view of staining. Chromic 

 acid and its salts, osmic acid, the chlorides of palladium, of 

 gold, and of iron, are reagents that seem to combine chemi- 

 cally with the tissues, and render necessary a special after- 

 treatment and special modes of staining, whilst picric acid, 

 nitric acid, and corrosive sublimate do not appear to enter 

 into that kind of combination, and can be entirely removed 

 from the tissues by washing, and leave the tissues in a state 

 in which they are susceptible of any kind of staining. 



Practically it amounts to this, that if you fix with a chromic 

 or osmic mixture, you cannot stain with carmine but can only 

 stain with hsematoxylin, if you wish to stain your objects in 

 toto ; or you may make sections, and stain them with safranin 

 or some other coal-tar colour. Whilst if you fix with subli- 

 mate or a picric acid mixture, you may do as you like in the 

 matter of staining. 



The after-treatment appropriate to each fixing agent is 

 indicated in the special paragraphs. 



23. Choice of a Fixing Agent. Indications concerning the 

 proper fixing agent to employ for the different tissues and 

 organs of the animal kingdom, will be found in Part II. The 

 following remarks are intended as hints for beginners only. 



The chief fixing agents for general work are FLEMMING'S 

 mixture, HEKMANN'S platinum-chloride mixture, osmic acid, 

 corrosive sublimate, and picro-sulphuric acid. 



I recommend that Flemming's mixture should be used 

 wherever it is possible, as I believe it to be in general by 

 far the best fixing agent yet invented, with the exception 

 of Hermann's mixture, which is unfortunately too expensive 

 to be employed in large quantities. 



But it will not always be found possible to use it. Its 

 low power of penetration, for instance, puts it out of court 

 in the case of very impermeable objects, such as are fre- 

 quently offered by the Arthropoda. For these, picro-sul- 

 phuric acid may be recommended. 



For very small objects, such as may be mounted whole, 

 osmic acid is nearly as good as Flemming's mixture, and 

 is frequently much more convenient to use. 



