DIATOMACEtE. 



[ 251 ] 



DIATOMACE^. 



containing the valves, when evaporated on a 

 slide, leaves no film (of calcareous salts) at 

 the margins of tlio drop. This is a some- 

 what tedious process ; but it is essential that 

 it should be tiioroughly carried out. If the 

 valves be not thoroughly washed, the film 

 of nitrate of lime remaining upon the slide 

 will absorb water from the atmosphere, and 

 the whole will be spoiled. 



It has been found advantageous, after the 

 treatment of the valves with acid and 

 washing, to shake them up with a mixture 

 of equal parts of gljxerine and water ; they 

 then sinlv through the water into the gly- 

 ceruie, the flocculeut matter being retained 

 by the water. Thorough washing and de- 

 cantation will then remove the glycerine 

 (Neil). Boiling with a small portion of 

 yellow soap, after the acid treatment, is 

 highly recommended (H. L. Smith). 



The appearance of the valves thus pre- 

 pared will var}' according to their structure, 

 and the manner in which they are examined. 

 In some cases the valves appear colourless, 

 and the markings perfectly distinct with 

 the ordinary direct light of the mirror, pro- 

 vided the power be sufficient (PI. 17. tigs. 

 2, 29; PI. ol. fig. 1). In others (PI. 15), 

 the valves appear coloured when viewed by 

 the ordinary light. But when the mirror is 

 brought to one side, and the light is thus 

 thrown upon the object obliquely, one or 

 two sets of fine parallel black lines are seen 

 traversing the valves (PI. 1. figs. 17, 18; 

 PI. 15. figs. 10, 12, 15, &c.). And when an 

 object-glass of considerable aperture is used, 

 with the condenser and central stop exactly 

 centrical (Introduction, p. xix), the lines 

 are replaced eutirelv or in part by a series of 

 black dots (PL 1. tig. IG; PI. 15. figs. 39,40, 

 &c.) ; these, under a high eyepiece, have 

 distinctly angular forms, sometimes appear- 

 ing regularly hexagonal (PI. 15. fig. 41). 

 If the condenser and stop be not exactly 

 centrical, or the surface of the valve be not 

 flat, the appearance of dark dots will be 

 replaced by that of a set of brilliant pearls 

 (PI. 15. fig. 46), which, as we have stated, 

 many observers consider to represent little 

 hemispherical elevations on the surface of 

 the valves ; or the true form of the dark 

 dots win be replaced by some other ; thus 

 hexagonal dots may be made to appear tri- 

 angular, quadrangular, &c., and those dots 

 which cannot be conceived to be really 

 hexagonal (PI. 15. fig. 39) may be made 

 to appear so. 



There can be little doubt that the valves 



of all the Diatomaceoe are furnished with 

 markings, although in some of thorn they 

 have not yet been detected. In the most 

 difficultly resolvable of those at present 

 known, lines only can be rendered evident, 

 although these probably consist of rows of 

 dots ; these very difficult valves require the 

 use of an Amici's prism (Introduction, 

 p. xxi), Powell's latest modified Gillett's 

 condenser, the method described in the 

 Introduction, p. xxx, with immersion- 

 lenses, or Wenham's reflex-iUuminator 

 (See also Illu^mination). 



We have already stated that we believe 

 the dots to consist of depressions. In re- 

 viewing the considerations relating to this 

 point, we may divide the valves into those 

 which exhibit the dots by ordinary light, 

 and those which require oblique light and 

 the use of stops. 



In those visible with ordinary light (PI. 17. 

 fig. 29 ; PL 51. fig. 1, &c.), the valves are 

 thinner and weaker at the parts occupied by 

 the dots, so that the line of fracture corre- 

 sponds to these parts and the depressions, are 

 distinctly visible at the edges of the cm'ved 

 portions of the valves (PL 17. fig. 2 b). In 

 those requiring the use of oblique light and 

 stops, the line of fracture also corresponds 

 to the rows of dots, provided the light be 

 equally oblique on all sides ; and the same 

 appearances are presented by the dots in 

 both cases, beginning with those in which 

 they are very large (as in Isthmici), to those 

 of moderate and small size (as in the species 

 of Coscinodiscus), down to those in which 

 they are extremelj^ minute (as in Qyrosigma 

 &c.). Moreover, analogy aftbrds a strong 

 confirmatory ground; for the Diatomacese 

 form a very natural family : and if the dots 

 are depressions in some genera, we might 

 expect them to be so in the others. 



The explanation of the manner in which 

 oblique light renders the dots visible, has 

 been given under Angular Aperture, 



The method of determining the structure 

 of the frustules of the Diatomaceaj is the 

 same as that of microscopic bodies in ge- 

 neral, and has been laid down in the In- 

 troduction, p. xxxviii. The presence or 

 absence of a gelatinous envelope or a stipes 

 should first be decided. The general form 

 of the frustides, both in the front and side 

 view, is next examined, which should be 

 done while they are immersed in water, 

 — the frustules being made to roU over by 

 gently moving the glass cover with the 

 point of the mounted needle, the eye being 



