116 PREPARATION AND MOUNTING 



AS warm as the hand can comfortably bear, will stop the 

 expansion, when the portion which we wish to mount should 

 be cut off from the mass of salt by simply scratching the 

 film around, and pure Canada balsam with the thin glass 

 used. Breathing upon the film, or allowing the slide to 

 become cold and attract the moisture from the atmosphere, 

 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 uncertain 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. Into the cause 

 of this we have no need to enter here, and as it has been 

 flsewhere discussed, I can 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 differ- 

 ence is effected even by the strength of the solution. There 

 are some crystals, also, which are called forth in insulated 

 portions, showing no formation upon the ground ; but even 

 when mounted in any preserving fluid, and unchanged for a 

 year, a new action seems to arise, and a groundwork is pro- 

 duced 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 VI. 



