146 FIXATION 



general conclusions can also be drawn. Some of these are shown in 

 table 9. It will be noticed what a preponderating place is taken in 

 our most familiar mixtures by the eight primary fixatives described 

 in detail in chapters 5 and 6. The only other fixative substances 

 that occur in any of these 25 mixtures are nitric acid, sulphuric 

 acid, chloroplatinic acid, trichloracetic acid, cadmium chloride, 

 and creosote. Of these, nitric acid occurs in two of the 25 mixtures, 

 each of the rest in one only. It will be remembered that the two 

 representative non-aqueous mixtures (Clarke and Carnoy) both 

 contain ethanol and acetic acid ; these substances therefore appear 

 respectively in 5 and 15 of the 27 chosen mixtures. 



The two occurrences of acidified potassium dichromate should 

 properly be added to the 8 of chromium trioxide, since, as we 

 have seen (p. 127), the ions are essentially the same. 



The pH range of the 25 aqueous mixtures extends only from 

 0*3 to 4-9. It may be remembered that one of the primary fixatives 

 (formaldehyde) is sometimes used in alkaline solution (p. 112). 



It is not the purpose of table 8 to help in the making up of the 

 various mixtures. It is best to keep stock solutions of most of the 

 primary fixatives at suitable concentrations and to make up mix- 

 tures from these. In cytological work one requires very small 

 amounts of fixatives. If a piece of tissue is only about a millimetre 

 in greatest diameter, 2 ml of fixative are amply sufficient: 4 ml are 

 enough for nearly all cytological purposes. It is convenient to 

 have stock solutions from which several useful fixatives can be 

 made up. Three valuable mixtures are Hermann, Flemming's 

 strong fluid with full acetic, and a fixative called Lewitsky-saline 

 (LS). The latter is Lewitsky's fiuid with the addition of sodium 

 chloride: it can be strongly recommended for studies of cyto- 

 plasmic inclusions.^^ These three fixatives can be made up from 

 stock solutions in the way shown in table 10. If these small amounts 

 of fixatives are used, the high cost of osmium tetroxide and chloro- 

 platinic acid no longer forbids their frequent use. Flemming's 

 fluid is not stable (though it does not change colour), for chromium 

 trioxide slowly oxidizes acetic acid; it is therefore always best to 

 make up very small amounts. 



Quite a number of commonly used mixtures are best made up 

 at the moment of use, since they contain ingredients that interact. 

 Some part of the mixture may generally, however, be made, and 

 the fixative will then be available at short notice. Thus for 

 Sanfelice's fluid the chromium trioxide must be kept separate 



