206 SELECTION AND USE 



or there is an unequal distribution of the constituents as re- 

 gards the substance itself and the surrounding fluid. Moreover 

 the fluids contained in many objects are displaced by the pre- 

 serving medium, and tend to dilute the latter. In most cases, 

 therefore, where the preserving medium is a liquid, the desired 

 result is best attained by soaking the substance in the fluid for 

 several days before mounting, changing the liquid two or three 

 times, and finally mounting in fresh fluid of regular strength. 

 We would lay great stress upon this point, having seen many 

 fine preparations spoiled by pursuing a different course. The 

 late Dr. Goadby, whose skill in this department was well 

 known, always insisted upon this course, and during a some- 

 what extended intercourse with him, and observation of his 

 methods and processes, we became fully convinced of its im- 

 portance. 



With many preservative liquids, it is well to begin with a 

 diluted article, and gradually increase the strength at each 

 change of fluid until the proper strength has been reached. 

 This course is specially recommended with glycerine and saline 

 solutions. 



Another point which demands attention is the entire exclu- 

 sion of air, especially of oxygen. Now air adheres with great 

 tenacity to most surfaces, such as those of glass or metal, and it 

 dissolves to a considerable extent in all watery solutions. To 

 get rid of it, the surface of the cell and cover should be either 

 well warmed, and then allowed to cool just before being filled, 

 or washed with alcohol (after which it may be dried). To ex- 

 pel the air from the liquids, they should be boiled, and to pre- 

 vent the absorbtion of a fresh dose of air, they should be kept 

 well stoppered. But as air will find access to the liquids so as 

 ultimately to saturate them, it is necessary to boil the fluids at 

 frequent intervals, so as to get rid of this element. Without 

 strict attention to these points it is almost impossible to pre- 

 serve animal substances for any length of time in saline fluids. 



Mounting Objects. For the purpose of conveniently 

 exhibiting and comparing objects, and arranging them in cab- 

 inets where they can be at all times accessible, it is necessary to 

 mount them securely in such a manner that they may be easily 



