322 MOUNTING MICROSCOPIC OBJECTS [Cn. X 



the preparation is to be studied for some time, it is better to avoid 

 currents and evaporation by painting a ring of castor oil around the 

 cover in such a way that part of the ring will be on the slide and part 

 on the cover (fig. 204). 



It cannot be too strongly emphasized that if one is to study living 

 or fresh tissues they must be mounted in a liquid which will not injure 

 them. The liquid in which they are naturally found is of course the 

 most nearly normal of any, and should be always used when possible. 

 Water seems a very bland and harmless liquid, but it has a very de- 

 cidedly injurious effect on living tissues which are normally bathed by 



the fluids of the body, for 

 forctji/ they always contain salts 

 and colloid material. Dis- 

 tilled water is more de- 



FIG. 190. FINE FORCEPS FOR HANDLING COVER- leterious than tap water 

 GLASSES AND OTHER DELICATE OBJECTS. because k contains no 



salts. It would be deleterious to water organisms, because all natural 

 waters contain a greater or less quantity of organic and inorganic 

 substances in solution. In examining water organisms use the water 

 in which they are found. If the water supply of a city or town has 

 a filtration plant the water is likely to be unsuitable for raising water 

 forms like salamander embryos, and the embryos of the frogs and 

 toads, besides many other water forms. One must take the trouble to 

 get the water from the natural breeding places if the embryos are to 

 be successfully raised in a laboratory. (See also 520-521, 584.) 



500. Permanent mounting. There are three great methods of 

 making permanent microscopic preparations. Special methods of 

 procedure are necessary to mount objects successfully in each of these 

 ways. The best mounting medium and the best method of mounting 

 in a given case can only be determined by experiment. In most 

 cases some previous observer has already made the necessary experi- 

 ments and furnished the desired information. 



The three methods are the following: 



(1) Dry or in air ( 501-504). 



(2) In some medium miscible with water, as glycerin or glycerin 

 , i jelly ( 505-509)- 



