148 SELECTION AND USE 



bodies which have accidentally found their way into a prepara- 

 tion have been mistaken for part of the specimen. The only 

 way to avoid similar errors is to exclude all such intruders by 

 means of proper covers, and to become familiar with them so 

 that they may be instantly recognized when present. Dr. Beale 

 gives the following list as those that are most apt to find their 

 way into the preparations of the microscopist : Oil globules; 

 milk; starch from the potato, wheat and rice; bread crumbs; 

 feathers; worsted; fibres of flax, cotton and silk of different 

 colors; human hair, cat's hair and hair from blankets; the scales 

 of butterflies and moths, particularly those from the common 

 clothes moth; fibres of wood, fragments of tea leaves, hairs 

 from plants, vegetable cellular tissue and spiral vessels; particles 

 of sand. The curious circumstances under which such bodies 

 will find their way into a specimen was recently illustrated in the 

 author's experience. In a liquid submitted for examination, 

 and said to be pure, he found foreign matter. It proved to be 

 brick dust, used to clean the tin funnel with which the vessel 

 was filled, and which had been washed in by the passage of 

 the fluid. The student can have no better exercise than to 

 examine these intruders and familiarize himself with their ap- 

 pearance. 



4 



Preservative Processes. The object of all preserva- 

 tive processes is to prevent any change either in the structure 

 or composition of the object. An object may be most perfectly 

 prepared and beautifully mounted, but if it be not so treated 

 as to preserve it from change, the labor thus expended is wasted, 

 as regards the preservation of a permanent record. And yet 

 how many objects there are that we would like to keep for 

 future examination and comparison, or to show to friends. 

 This department of the treatment of objects is, therefore, of 

 great importance, and success in it can only be obtained 

 through a thorough understanding of the principles involved. 



There are four methods in common use for the preservation 

 of perishable animal and vegetable substances: 1, Constant ex- 

 posure to temperature considerably below the freezing point of 

 water; 2, the perfect exclusion of air; 3, reduction to a state of 

 complete dryness; and 4, the employment of certain anti-septic 



