REAGENTS. 277 



decalcifying the armor of crustaceans, the shells of mollusks, and the 

 bones of vertebrates. 



Nitric Acid. This acid may be used for decalcifying, but it is 

 more expensive than hydrochloric. A 10-per-cent solution is useful 

 for macerating other tissues preparatory to a dissection of the ner- 

 vous system. If the bodies of small animals are kept in it for two 

 or three days, its destructive action on other tissues will leave the 

 nerves more easily accessible. Strong nitric acid is used for isolating 

 the spicules of fresh-water sponges. 



Glycerine. This is a very useful fluid in which to examine small 

 objects microscopically. A 25-per-cent solution greatly increases the 

 transparency of small objects soaked in it, making the structure more 

 evident. 



Canada Balsam. A sirupy solution, made by dissolving the dried 

 resin in turpentine or xylol, is an invaluable medium for making 

 permanent mounts of microscopic objects. Both air and water must 

 first be removed from the objects to be mounted in it. Hard, diy 

 objects, such as the claws of insects and the isolated spicules of 

 sponges, may be placed in a drop of balsam on a slip, heated gently 

 over an alcohol lamp or Bunsen burner to expel all air, and covered 

 with a thin cover slip ; but a less direct method must be employed 

 in mounting soft tissues. These must be covered first with weak, 

 and afterward with strong, alcohol, to remove the water they contain. 

 They must then be removed from alcohol into turpentine or xylol 

 for clearing. They may then be placed in a drop of balsam on a 

 slide, and at once covered. 



Chloroform is very useful for removing excess of balsam from slips. 



Alcohol. The use of alcohol in dehydrating tissues for mounting 

 in balsam has already been stated. But one other of its many uses 

 in the laboratory will be mentioned here, its use in preserving 

 specimens. In this use its action is the same as in the preparation 

 of specimens for mounting. It preserves objects by removing the 

 water from them. It removes the water by osmosis, and it is there- 

 fore weakened every time it is used by just so much water as it takes 

 from the tissues. Two grades of alcohol should ordinarily be used 

 for preserving specimens, a weaker grade, in which the dehydration 

 is begun, and a stronger grade, into which they may be transferred 

 from the weaker, for permanent preservation. Alcohol of 70 to 80 per 

 cent strength, after osmotic action has ceased, is strong enough for 

 permanent preservation. 



Turpentine, Xylol, Chloroform, Ether, etc., are used in their com- 

 mercial form, as directed in the text. 



