FIXING AND HARDENING. 25 



immediately precipitated by them if the solution is rendered 

 acid. " Many kinds of cell contents/' he says (op. cit., 

 p. 10) " indeed the majority, have an alkaline reaction, and 

 are thereby quite inaccessible to the precipitating action of 

 certain agents, such as osmic acid, or bichromate ; and the 

 action of certain other fixatives, such as platinum chloride 

 and chromic acid, is more or less hindered by the presence 

 of free alkalies. For neither the chromic acid (of solution of 

 F lemming) nor the platinum chloride (of solution of Hermann) 

 would be adequate to act as acidifiers to the osmic acid of 

 the mixtures. They cannot do so, firstly, because they them- 

 selves become combined with the tissues much more quickly 

 than the slowly working osmic acid, and secondly, because 

 they themselves have only an extremely weak acid reaction/ 7 

 Hence the function of the organic acid is to bring into play 

 the precipitating power of the other ingredients. 



Not only is it true that the most complete fixations can 

 only be obtained by means of mixtures, but it is also true 

 that no one mixture can serve all ends. It is probably mis- 

 leading to recommend this or that reagent as " the best for 

 general purposes," or the like. The following suggestions, 

 however, may be helpful to the beginner. 



In Part II will be found mentioned the fixatives that 

 appear the most appropriate for different purposes (embryo- 

 logical, cytological, etc.), and for the different tissues and 

 organs, etc. 



Speaking generally, I think the beginner should avoid 

 such things as liquid of FLEMMING and similar mixtures. He 

 may take, instead, liquid of TELLYESNICZKY. This gives a fair 

 fixation and is easy to manage, but it is wanting in pene- 

 tration. 



( 'ormsive sublimate is a good all-round fixative, with 

 excellent penetration, but is not quite so easy to manage. 



Picric acid gives a fair though weak fixation, with very 

 good penetration, is easy to manage, and does not make 

 ti><ues brittle, which sublimate easily may do. 



To ensure the best results, all fixative* should be acid, for 

 the reasons explained above. They may conveniently be 

 made to contain from one to five per cent, of acetic acid. 



The student may consult with advantage, besides the work 

 of FISCHEK quoted 27 'a, the paper of TELLYESNICZKY, Arch. 



