44 GENERAL HISTORY OF THE II^FUSOEIA. 



I am, however, quite at a loss to imagine how the same principle would apply 

 on the hypothesis that the dots are depressions, nor do I see upon what 

 principle the phenomenon is explicable." 



A (hrect demonstration that the markings in general of the Diatomea3 are 

 elevations is attempted by Mr. Wenham, whose knowledge of optics and prac- 

 tical skill in mechanical manipulation are not exceeded by any microscopist 

 of the present day {J. M. S. 1855, j)p. 244^245). To quote his words— "A 

 careful study of the coarser varieties will distinctly prove that the markings 

 are raised ribs or prominences on the surfaces, in some instances on one side 

 of the shell only, as seen in the Cajnpylodiscus spiralis and others. Though 

 the microscope proves this fact satisfactorily in the large species, it fails to do so 

 in the most difficult specimens, chiefly on account of the above-named deceptive 

 appearances, arising from the irregular refraction and reflection of light. It 

 occurred to me that it might be possible to obtain a perfect cast or impression 

 of the structure ; and by viewing this as an opake object, the error, if arising 

 from refraction, would be avoided, and a discovery might be the reward of the 

 experiment. I have succeeded in effecting this by means of the electrotype 

 process, which for many reasons is to be preferred, as it does not distort the 

 object, and is so minutely faithful that even the mere trace of organic matter, 

 left by a slight finger-mark, is perfectly copied. The method I have adopted 

 is this : — Procure a small plate of metal highly polished (a piece of daguerreo- 

 type plate answers extremely well), and, after gently heating it, rub a piece 

 of bees- wax over the surface ; while this is still melted wipe it nearlj^ all off 

 again with a piece of rag, so as to allow a very thin film to remain ; when 

 the plate is cold, arrange the Diatomacea or other objects, previously moist- 

 ened, upon the waxed surface, heat the plate again to at least 212°, in order 

 to cement the objects on it. The wax serves a twofold purpose : first, its 

 interposition prevents the possibility of a chemical union of the metallic 

 deposit with the plate ; and secondly, the object is secm^ely held thereto by 

 its agency. The objects are now ready to receive a coating of copper. If 

 the battery is in good working order, three or four hours will give a film 

 sufficiently strong to bear removal ; when this is stripped off, if the process 

 has been properly managed, the objects will be seen imbedded in its siu-face ; 

 whether they are silicious or organic, they may be entirely dissolved out by 

 boiling the cast in a test tube Avith a strong solution of caustic potash, and 

 afterwards washing with distilled water ; the copper film may then be mounted 

 in Canada balsam. By these means I have obtained distinct impressions of 

 the markings of some of the more difficult Diatomacea, such as N. (Pleuro- 

 sigma) Balticum, P. Hipjpocam'pus, &c., leaving no doubt of their prominent 

 nature." (See MicroscojJic Cabinet, ed. 1832, chap. xvi. and xviii.) 



Besides the superficial markings explicable on the supposition of an invest- 

 ing areolar membrane, and the sculptiu^ing of the silicious epiderm, there 

 are others, dependent on structural modifications of the sihcious laminae of 

 the valves, and on inflections of these internally. Among the former are 

 many of the stronger-marked costae and pinnules of Ehrenberg ; and among 

 the latter are to be reckoned the imperfect partitions (^ septa') seen in 

 several genera, and those peculiar processes of the internal surface which 

 Kutzing called ' vittce.' Schleiden described ' pinnules ' to be clefts or fissures. 

 " In these spots (says he), the shield consists of two leaves lying one over the 

 other; these leaves are penetrated by the small clefts, which, when both 

 the lamellae touch each other, are somewhat broader, which explains the 

 varying breadth of the clefts according to the alteration of the foci. Frag- 

 ments in which this structui'e is clearly represented may be frequently obtained 

 by crushing the shield." (XVI. 5, 6.) 



