COLLECTION OF SPECIMENS 175 



necessary, for any reason, to remount the slide. 

 Another effect which the formalin has on the 

 gelatine is to cause it to shrink considerably, and 

 this may have, and certainly sometimes has, a 

 detrimental effect on the leaves of the plant in the 

 cell. Moreover, it would seem as though the 

 shrinkage of the gelatine, under certain conditions, 

 tended to force out some of the glycerine in the 

 jelly, for recent experiments have shown that, 

 with slides that have been allowed to lie in an 

 unusually cold situation, those in which formalin 

 has been used are the first to develop beads of 

 glycerine round the edge of the cover-glass. 

 While therefore, at first, formalin appeared to 

 offer very decided advantages, a closer acquain- 

 tance with it has convinced me that it is not a 

 desirable medium to use in this way. 



Although it is often quite impossible to assign 

 any specific reason for a slide having gone wrong, 

 I have found that the observance of the following 

 few simple rules greatly minimises the risks of 

 failure : 



1. Care should be taken to free the object, as 

 far as possible, from the preparatory fluid before 

 immersing it in the jelly ; hence the advisability 

 of using two quantities of jelly, as before sug- 

 gested, for this helps to remove all traces of the 

 fluid. 



2. No pressure whatever should be applied to 

 the cover-glass when placing it in position on 



