XXXU INTRODUCTION. 



in such a medium, as the presence of the cell-contents would 

 prevent the access of the balsam to the interior of the frustule. 

 To procure the valves in a separated state, it is necessary to boil 

 the frustules for a few minutes in muriatic or nitric acid ; thus 

 removing the softer parts, and dissolving the connexion between 

 the various portions of the frustule. The acid must afterwards 

 be removed by repeated washings, allowing the siliceous mate- 

 rials to settle to the bottom of the vessel after each addition of 

 water : we thus procm-e the specimens in a proper state. 

 Having pom^ed oiF the superfluous water, a di'op of the fluid 

 containing the Diatomacese is to be placed on the slide, evapo- 

 rated to dryness, and the balsam and cover applied as in the 

 ordinary mode. To avoid the presence of air-bubbles, it is only 

 necessary to use the balsam in a sufficiently fluid state ; and this 

 may be secured by mixing it with ether or rectified spirits 

 of turpentine, taking care entirely to expel the latter by the 

 apphcation of a gentle, but long-continued heat, before applying 

 the glass cover. 



With some valves, more especially in those specimens the 

 resolution of whose cellular structure requires the application of 

 the highest powers of the instrument, a different mode of mount- 

 ing is necessary, as the application of balsam obliterates the more 

 delicate markings. Such objects must be preserved in a dry 

 state, without being immersed in any medium, and must not 

 only be covered with glass sufficiently thin to allow the appi-oach 

 of a lens of high power, but also placed upon a glass slide, 

 whose thickness will not impede the use of condensing lenses 

 of short focal lengths. These objects are accomphshed by per- 

 forating a wooden or metallic slide, and placing upon the per- 

 forations, which ought to be as large as practicable, a portion of 

 thin glass, to receive the object, cementing this to the slide, and 

 placing over it a cover of glass not exceeding xiot^i of an inch 

 in thickness, and thinner if possible. To prevent the admission 

 of moisture, which would ultimately make its way to the object 

 and destroy its value, it is indispensable that the cover should 

 be cemented to the thin glass below. A neglect of this precau- 

 tion will, sooner or later, be followed by the loss, to all valuable 

 purpose, of the contained specimens. 



