14 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 chemically 

 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 

 sublimate 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. 



24. 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 ( 35 and 36), 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. 



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 frequently 

 offered by the Arthropoda. For these, picrosulphuric 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. 



For objects a little larger, and for much embryological 

 work on objects that it is convenient to have stained in toto, 



