24 FIXATION 



two reasons. First, for simplicity: it would be tedious to have to 

 mention all the products of solution every time. Secondly, for 

 accuracy in the statement of concentrations. If, for instance, one 

 adds distilled water to 0-5 g of chromium trioxide to make 100 ml 

 of solution, the latter should not be called a 0-5% solution of 

 'chromic acid'; for the oxide takes up water in ionizing, and 

 the acid produced (cations plus anions) is present at more than 



0-5 %• 



In the experiments described in the succeeding chapters, the 

 substances were always used at the concentrations given in the 

 list, except where the contrary is distinctly stated. It will therefore 

 be possible to omit mention of concentration and thus avoid 

 unnecessary repetition. 



Non-fixative or 'indifferent' substances, such as sodium 

 chloride or sodium sulphate, are often added to solutions of these 

 fixatives. The effects of this will be considered in a later chapter 

 (p. 80). 



None of the listed substances has all the qualities of a perfect 

 fixative, and mixtures are therefore made with the intention of 

 combining the virtues of the ingredients. Most mixtures contain 

 two or more of the listed substances, and often an indifferent 

 substance as well. The principles that should guide the design of 

 fixative mixtures will be considered in chapter 7 (p. 139). 



Mixtures are generally called by the names of those who intro- 



