22 CHAPTER III. 



constituents of tissues, always a phenomenon of precipitation. 

 The more solid constituents (such as fibrils that are visible 

 during life, nucleoli, and the like) he admits may be acted on 

 by fixing reagents without the formation of any visible pre- 

 cipitates. But all the liquid ones, in so far as they are 

 fixed at all, are visibly precipitated in special precipitation 

 forms, which vary according to the precipitant. Each fixing 

 agent gives its own characteristic fixation image, which may 

 be more or less lifelike, but can never be absolutely so. 

 Fischer gives copious descriptions of the precipitation 

 forms of the chief organic compounds found in tissues, and 

 of the precipitation powers of the chief fixing agents, which 

 the reader will do well to study. 



It seems to be a consequence of Fischer's theory of fixa- 

 tion by precipitation that the most energetic fixing agents 

 should always be found amongst the most energetic precipi- 

 tants. But on the showing of his experiments this is not so. 

 For instance, it is allowed on all hands that osmic acid is a 

 most energetic fixative. But Fischer finds (op. cit., pp. 12 

 14, 27) that it is a very incomplete and weak precipitant. 

 Or, to take a contrary instance, he finds that picric acid is 

 an energetic precipitant of the majority of cell constituents ; 

 but surely every cytologist must admit that it is a very 

 incomplete fixative. 



It would seem to follow, from these instances and from 

 other similar ones, that Fischer 's tables of precipitating 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 pre- 

 cipitates, 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 

 observations and reasonings 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. His work 

 is deserving of most careful study. 



