INTRODUCTION. 



acquainted with their general appearance, it remains for me to 

 point out the method to be observed in their collection, and to 

 furnish the student with a few directions as to their subsequent 

 preservation. 



Let him provide himself in the first place with the necessary 

 apparatus. For the field, this includes a good stock of small 

 wide-mouthed bottles, that each gathering may be kept perfectly 

 distinct ; a long rod or stick, to which can be attached a small 

 muslin net ; a cutting hook, of about three inches in length ; 

 and a broad flat spoon : the first, to collect such specimens as 

 float upon the surface, or are held in suspension by the w^ater ; 

 the second, to remove the larger Algae which may be covered 

 with parasitic Diatoms ; and the third, to skim the surface of the 

 mud for those which lie at the bottom of the pool. 



He will probably flnd, notwithstanding every care, that his 

 specimens are mixed with much foreign matter, in the form of 

 minute particles of mud or sand, which impair their value, and 

 interfere with observation, especially with the higher powers of his 

 instrument. These substances the stndent may remove in various 

 ways ; by repeated washings in pure water, and at the same 

 time, profiting by the various specific gravities of the Diatoms 

 and the intermixed substances, to secure their separation ; but 

 more particularly, by availing himself of the tendency which the 

 Diatomacese, in common with all gro"\Adng plants, possess, of 

 making their way towards the light. The free forms may be 

 thus procured in a tolerably clean state ; all that is necessary 

 being, to place the gathering which contains them in a shallow 

 vessel, and leave them undisturbed for a sufficient length of 

 time in the sunlight, and then carefully to remove them from 

 the surface of the mud or water. 



Having performed these operations, which a little practice will 

 render comparatively easy and generally successful, the next pro- 

 ceeding is to preserve the specimens in such a manner as to 

 render them suitable for examination by the microscope at any 

 future time. This may be done in various ways, according to 

 the nature of the species and the precise object desired. 



The simplest method, and the one most generally useful to 

 the scientific observer, is simply to dry the specimens upon small 



