138 FIXATION 



unfavourably upon them, not the hydronium ion ; for as Casselman 

 and Jordan ^^* showed, mitochondria can be quite well seen in 

 paraffin sections of tissues fixed in o-iN hydrochloric acid. 



Compatihility with other fixatives. Acetic acid is compatible with 

 all other fixatives, but when it is mixed with potassium dichromate, 

 the fixation-image of chromium trioxide is given. 



Unclassified remarks. The fixation- image given by acetates in 

 paraffin sections is dependent mainly on their pH. This was 

 shown by Zirkle,^^'^ who experimented with various salts con- 

 taining the same amount of the acetate ion as 2% acetic acid. 

 Those acetates that gave a pH less than 4-0 (bismuth subacetate, 

 for instance) tended to produce the characteristic 'acid' fixation- 

 image of acetic acid. Sodium acetate, on the contrary, and others 

 that also gave a less acid pH than 4-0, generally macerated tissues. 

 In some cases the cation affected the image. 



The other short-chain fatty acids (formic, propionic, butyric, 

 valeric) all give much the same fixation-image in paraffin sections 

 as acetic; so do glycollic, glyceric, lactic, and gluconic. ^^^ Tri- 

 chloracetic acid, however, acts in an entirely different way. It 

 is a coagulant fixative; it leaves the nucleolus readily colourable 

 by iron haematein, and mitochondria can be well fixed by mix- 

 tures containing it at 2%.^^^' ^^^ 



