22 rilAl'TKK III. 



power cannot be taken as a measure of the fixing power of 

 the reagents. And further, the study of the fixation images 

 of tissues afforded by osmic acid, formaldehyde, and other 

 reagents, seems to show that the coagulation brought about 

 by them is in part accompanied by the formation of visible 

 precipitates, but in part not so, and that they may do their 

 work to a larger extent than he seems to admit through a 

 homogeneous coagulation. But from his very suggestive obser- 

 vations it certainly appears that the formation of visible 

 precipitates is a very wide-spread, if not universal concomitant 

 of fixation ; and that the wider the precipitating power of a 

 fixative (i.e. the greater the number of organic liquids that 

 it can precipitate), the greater will be the number of artefacts 

 to which it can give rise. 



30. The Characters of the Usual Fixing Agents. A good 

 fixing agent should first of all preserve all the elements it is 

 desired to fix. But that is not enough; it should also give 

 good optical differentiation, and should have sufficient power 

 of penetration to ensure that small pieces of tissue be equally 

 fixed by it throughout. No single substance or chemical 

 compound fulfils all that is required of a good fixing agent ; 

 hence it is that all the best fixing agents are mixtures. 

 Osmic acid, for instance, fulfils some of these conditions, but 

 not all of them. It kills rapidly and preserves admirably 

 the elements of cytoplasm, but nuclei not so well. But the 

 optical differentiation that it gives, though sometimes good, 

 is often very inferior. For osmic acid, by coagulating in 

 nearly equal degrees alike the spongioplasm (the plastin 

 reticulum) and the hyaloplasm (the enchylema) of the cell- 

 body, and the chromatin of nuclei, raises alike the refractive 

 indices of all of them ; so that if the fixing action have been 

 in the least degree overdone, the cells acquire a homogeneous 

 aspect in which the finer details are obscured by the general 

 refractivity of the whole. If now, instead of using it pure, 

 it be used in combination with acetic acid, a better differentia- 

 tion is obtained; for acetic acid, whilst enhancing, or at all 

 events not interfering with the fixation of chromatin, serves 

 to facilitate penetration and to counteract the excessive action 

 of the osmic acid on the protoplasm, so that the cells come 

 out less homogeneous and with more detail observable in 



