DIATOMACE.E. 



[ 252 ] 



DIATOMACE^E. 



kept upon the object, and a somewhat low 

 power used. 



The frustules should then be prepared, 

 and examined when dry as to their mark- 

 ings. Perhaps these may be visible by 

 ordinary light ; if not, the mirror should 

 be turned on one side as much as possible, 

 to obtain the effects of oblique light. If 

 lines then become visible, it does not 

 follow that the valves are marked with 

 lined structures such as grooves or ridges, 

 because the shadows of rows of dots may 

 become extended into lines under oblique 

 illumination in any direction in which the 

 dots will form a linear series. This point 

 must, however, be decided by examination 

 with the aid of the condenser, stops, &c. 



In using very oblique unilateral light, 

 spurious rows of parallel lines are often 

 seen, not only upon the valves of the Dia- 

 tomacese, but upon objects not possessing a 

 lined structure, as many crystals &c. These 

 can only be distinguished from those con- 

 nected with the presence of dots, by their 

 not being resolvable into dots, their greater 

 coarseness, and their variability in number 

 in a given space under different kinds of 

 illumination. 



If the direction of the lines changes with 

 the variation of the position of the valve to 

 that of the incident light, it may be pretty 

 surely predicted that the lines are spurious, 

 and that the condenser and stops will effect 

 their resolution into dots. 



The prepared valves of the Diatomacese 

 frequently appear coloured when dry, the 

 colour vanishing when they are moistened. 

 This colour arises from iridescence, and 

 not from the presence of pigment or 

 other colouring - matter (INTRODUCTION, 

 p. xxxiv, 3). 



Collection. In collecting the Diatpmaceae, 

 a number of phials (1- to 2-oz.), with wide 

 mouths and furnished with corks, must be 

 provided, in which they may be brought 

 home. The mouth of the bottle being closed 

 with the thumb and brought as closely as 

 possible to masses of them in the water, 

 on removing the thumb, the water will 

 enter and carry the Diatomacese with it 

 into the bottle. A spoon is frequently of 

 use in removing layers of them from the 

 bottom of the water, or from pieces of 

 woodwork &c. immersed in the water. 

 Many of them are entangled in the meshes 

 of Confervae and other Algae, or on the 

 submersed stems of the higher plants; these, 

 if fixed to the stems, can only be removed 



with them ; if, however, the masses of 

 Diatomacea& are merely entangled in the 

 meshes of their stems, they may be detached 

 and collected in the " ring-net " (INTRO- 

 DUCTION, p. xxviii), and the pieces of muslin 

 placed in the bottles. A stick with a loop 

 of string at the end, is often useful in pro- 

 curing those which would be otherwise 

 beyond reach : the neck of the bottle being 

 engaged in the loop, and the mouth kept 

 downwards when immersed in the water 

 until opposite and close to the masses of 

 Diatomaceae, it is then inclined upwards 

 and filled. On exposing the bottles to the 

 light for some hours, the Diatomaceae will 

 collect on the surface of the mud or other 

 matters, and can then be removed with a 

 dipping-tube. It is often difficult to free 

 them from minute particles of sand ; this 

 may, however, generally be done by dif- 

 fusing the deposit through distilled water, 

 allowing the mixture to stand for a short 

 time, and then pouring off the uppermost 

 portions; the sand being the heaviest, will 

 subside first. The deep-sea species may be 

 obtained by dredging, or by treating the 

 alimentary canal of fishes, mollusca, &c. 

 with strong nitric acid as above directed. 



The Diatomaceee are often found fossil ; 

 occurring in vast numbers in freshwater 

 and marine geological deposits, forming 

 hills, rocks, and various strata ; also in peat- 

 beds, fossil polishing powders, as tripoli, 

 berg-mehl, &c. The deposits from Fran- 

 zenbad, Bilin, Richmond (U.S.). San Fiore 

 (Tuscany), Bermuda, Lough Morne (Ire- 

 land), Peruvian guano, &c., are well known 

 as containing many of the most beautiful 

 species, and are sold by the dealers in mi- 

 croscopic objects and apparatus. 



The masses may sometimes be disinte- 

 grated by placing lumps in a test-tube, 

 covering them with Liquor Potasses, boiling 

 for a short time until the whole breaks up 

 into a mud, and then instantly pouring it 

 into a quantity of distilled water, and well 

 washing. 



Preservation. The Diatomaceas may be 

 preserved either in the dry state, immersed 

 m balsam, in water, or dilute spirit one to 

 six (PRESERVATION). For exhibiting the 

 delicate markings, they should be mounted 

 in the dry state, placed upon and covered 

 by the thinnest glass which can be obtained. 



The mounted sable-hair or bristle will be 

 essential in isolating single valves (INTRO- 

 DUCTION, p. xxvi) for mounting. 



With regard to the systematic arrange- 



