252 TEE MICROSCOPE. 



sharp penknife, and then, with two small pairs of forceps, 

 tearing asunder the two parts ; the upper surface, which 

 is the part to be mounted, must then be dried and placed 

 for a short time in spirit of turpentine, previously to 

 being mounted in balsam. If well prepared, the colour 

 of the petal will be almost as fresh as in its natural state. 

 In mounting diatomaceae, to show them as nearly as 

 possible in their natural condition, they should be first 

 well washed in distilled water and mounted in a medium 

 composed of one part of spirits of wine to seven parts of 

 distilled water. The siliceous coverings of the diatoms, 

 however, which show such beautiful forms under the 

 higher powers of the microscope, require considerable 

 preparation. The guano, or infusorial earth containing 

 them, should first be washed several times in water till 

 the water is colourless, allowing sufficient time for pre- 

 cipitation between each washing. The deposit must then 

 be put into nitro-hydrochloric acid (equal parts of nitric 

 and hydrochloric acids). A violent effervescence takes 

 place, and when this has subsided, the whole should be 

 subjected to heat, nearly but not quite up to the boiling- 

 point, for six or eight hours. The acid must now be care- 

 fully poured off, and the precipitate washed in a large 

 quantity of water, allowing some three or four hours 

 between each washing, for the subsidence of some of the 

 lighter forms ; the sediment must be examined under 

 the microscope with an inch object-glass, and the siliceous 

 valves of the diatoms picked out with a coarse hair or 

 bristle. 



"When it is desired to exhibit one of the larger forms, 

 it can be placed separately in the middle of a glass-slide, 

 and mounted in Canada balsam ; or when it is wished to 

 mount several forms at once, the fluid containing the 

 skeletons should be gently shaken, and a drop spread over 

 the centre of a slide, the glass being held over a spirit- 

 lamp till the fluid has all evaporated, when the object may 

 be either dry-covered or mounted in balsam in the usual 

 way, according as its structure may require the one or the 

 other treatment. Diatoms may be kept in distilled water 

 in a small corked phial for any length of time. 



For vegetable preparations, distilled water in which 



