EXAMINATION OF MARINE OBJECTS 97 



the thinnest then selected for examination in water as previously 

 described. 



Other objects are so soft that the cutting of sections becomes 

 impossible without previously hardening them. Methylated spirit 

 is a good hardening reagent, and many of the soft structures that 

 have been preserved in this fluid, especially if it has been used 

 undiluted, will be found sufficiently hard for cutting thin sections. 

 Among the other hardening reagents used by microscopists may 

 be mentioned a solution of chromic acid one part by weight of the 

 solid acid dissolved in from one hundred to two hundred parts 

 of water, and a solution of bichromate of potash one part of 

 the bichromate to about forty parts of water. In either case the 

 hardening of the object takes place slowly, and it should be 

 examined from day to day until the necessary consistence has been 

 obtained. 



The structures of many soft animals can never be satisfactorily 

 hardened for section-cutting by either of the above reagents, and 

 thus it becomes necessary either to freeze or to imbed them. In the 

 former case the object is first soaked in gum water a thin solution 

 of gum arabic and then frozen by an ether spray or by a mixture 

 of ice and salt. The sections should be cut with a razor just as the 

 object is beginning to thaw, and they may then be examined under 

 a cover-glass, in a drop of the gum water. 



The other method is conducted as follows: The soft -object 

 is first soaked in absolute alcohol to extract all the water it contains, 

 and is then transferred to paraffin that has been heated just to its 

 melting-point by standing it in warm water. After the object is 

 thoroughly permeated with the paraffin, the whole is cooled quickly 

 by immersion in cold water. Sections are now cut, the paraffin 

 being sliced away with the substance it contains. These sections 

 are placed in warm turpentine, where they are allowed to remain 

 until the whole of the wax has dissolved, and they may then be 

 mounted in a drop of turpentine, and covered with a cover-glass. 



We have given brief instructions for temporary mounting only, 

 but most amateur microscopists would undoubtedly prefer mounting 

 their objects permanently, so that they may be set aside for study 

 at any future period. Hence we append a few directions to this 

 end, advising the reader, however, to consult a work dealing 

 especially with this subject if he desires to become proficient in the 

 preparation of microscopic slides. 



Moist objects, including those which have been preserved in 



H 



