OP THE MICBOSCOPE. 199 



compound. The third and fourth are the methods usually em- 

 ployed in microscopy, but the same principles which render the 

 second method so successful in the preservation of canned fruits 

 and meats, deserve the attention of the microscopist. 



Drying, as a preservative process, can be applied to but few 

 specimens, chiefly transparent insect preparations, and opaque 

 objects. Blood and similar matters are also sometimes pre- 

 served by drying. Such preparations are so easily dried that 

 no special directions are needed. Warming them over a lamp, 

 or preferably on a water-bath, before applying the thin glass 

 cover (as directed in the section on mounting objects) is al- 

 most always sufficient. Where the specimen is liable to be in- 

 jured by heat it may be dried by placing it over sulphuric acid, 

 and covering both acid and preparation with a bell jar having 

 ground edges and resting on a perfectly flat plate of glass. The 

 acid soon absorbs all the moisture and renders the object perfectly 

 dry. Where a cell is used for an opaque object, and dryness is 

 essential, great care must be taken to make the cell impervious 

 to air, otherwise dampness will be sure to penetrate, and if the 

 object be of animal or vegetable origin, fungi will be very apt to 

 grow on it. We have found cells of cardboard peculiarly 

 liable to this defect, and such cells should always be thoroughly 

 saturated, and coated with varnish, such as gold size or Canada 

 balsam. 



The great dependence of the microscopist, however, is in the 

 employment of certain preservative media, of the most impor- 

 tant of which, the following is a list: 



CANADA BALSAM. Of all the media employed for the mount- 

 ing and preservation of objects, Canada balsam is undoubtedly 

 the most generally useful, and it is probable that more objects are 

 mounted in this material than in all the other media put together. 

 As a preservative it is perfect, and its action in rendering many 

 objects transparent and clear is often of great value. Frey tells 

 us that " several sorts of Canada balsam occur in commerce. 

 To be good it should be of thick consistence, nearly colorless, 

 and thoroughly transparent." One difficulty, however, is that 

 much of the Canada balsam that is sold is factitious, being 

 made of cheap resins dissolved in impure turpentine. Such 



