76 PREPARATION AND MOUNTING 



the slide to become cold and attract the moisture from the atmo- 

 sphere, will cause the crystallization to extend, and sometimes 

 greatly rob the effect ; so it is necessary to mount quickly when 

 the desired forms are obtained. As the crystals are very uncer- 

 tain as to the place of their formation, I may here mention that 

 they may be got in any part of the slide by piercing the film with 

 a needle-point ; but in some degree this necessarily interferes with 

 the centre. As the cause of this has no need to be entered into 

 here, and has been elsewhere discussed, I shall only give the above 

 directions, and say that there is a great field in this branch of study 

 which the microscope alone has opened. 



It would be useless to enter into particulars respecting the 

 various salts and treatment they require, as a great difference is 

 effected even by the strength of the solution. There are some crys- 

 tals, also, which are called forth in insulated portions, showing no 

 formation upon the ground ; but even when mounted in any pre- 

 serving fluid, and unchanged for a year, a new action seems to arise, 

 and a groundwork is produced which bears little resemblance to 

 the original crystal. Sometimes this new formation adds to the 

 beauty of the slide ; in other cases the reverse is the result, the 

 slide being rendered almost worthless. This action, I believe, 

 frequently arises from some liquid being contained in the balsam 

 or other mounting medium used ; and this is rendered the more 

 probable by the crystallization being called forth in an hour after 

 the balsam diluted with chloroform is employed, whereas no 

 change would have taken place for months (if at all) had pure 

 balsam been used. 



Sections of some of the salts are very interesting objects, but 

 the method of procuring these and their nature will be described 

 in Chapter V. 



The scales of various fish have been before mentioned as mount- 

 ed " dry ;" when, however, they are required for polarising objects 

 they are generally mounted in balsam, and some few in liquid. 

 The former method will be considered here. 



The eel affords a beautiful object for this purpose. The scales 

 are covered by a thin " skin" which may be slightly raised with 

 a knife and then torn off', in the same manner as the covering of 

 the geranium and other petals, described in Chapter II. The re- 



