OF MICROSCOPIC OBJECTS. 65 



Some of the large shells are destroyed by this method, but none 

 that are fit for microscopic use. An oblique light shows these ob- 

 jects best. 



These are sometimes treated in the manner described in Chap- 

 ter II. where the diatoms are spoken of, but many forms are liable 

 to be injured by this severe process. 



It has been before stated that some of the zoophytes may be 

 mounted dry, and others examined as opaque or transparent ob- 

 jects according to their substance. They are very interesting 

 when examined in the trough whilst living, but to preserve many 

 of them for future examination they must be mounted in some 

 preservative medium. Sometimes this may be one of the liquids 

 mentioned in Chapter IV., but if possible they should be kept in 

 balsam, as there is less danger of injury by accident to this kind of 

 slide. This method of mounting presents some difficulties, but I 

 think that all agree as to the trustworthiness of Dr. Golding Bird's 

 information on the subject, which appeared in the Microscopic 

 Journal. Of this, space forbids me to give more than a condensed 

 account, but I hope to omit nothing of moment to the reader for 

 whom these pages are written. 



After stating that there are few who are not familiar with these 

 exquisite forms, and have not regretted the great loss of beauty 

 they sustain in dying, he informs us that from their so obstinate- 

 ly retaining air in the cells and tubes when dried, it is hardly 

 practicable to get rid of it ; and they also shrivel up very seriously 

 in the process of drying. The following plan, however, he has 

 found almost faultless in their preparation. 



To preserve them with extended tentacles, they should be 

 plunged in cold fresh water, which kills them so quickly that 

 these are not often retracted. The specimens should be preserved 

 in spirit until there is leisure to prepare them ; if, however, they 

 have been dried, they should be soaked in cold water for a day or 

 two before being submitted to the following processes : 



1. After selecting perfect specimens of suitable size, immerse 

 them in water heated to about 120 and place them under the 

 receiver of an air-pump. Slowly exhaust the air, when bubbles 

 will arise and the water appear to be in a state of active ebullition. 

 After a few minutes re-admit the air and again exhaust, repeating 



4* 



