Dr. G. C. Walllch on the Diniomacea. 13 



We have therefore before us an answer to the problem, why 

 these organisms appear near the surface more readily in calm 

 and bright weather. It is only during such conditions of the 

 atmosphere that the refractive power of the medium they inhabit 

 remains undisturbed by the surface-tumult ; the rays of light 

 and heat penetrate freely into the depths, and produce, by their 

 combined influence, the amount of development under which 

 the ascent to the surface takes place*. 



It will be obvious that buoyancy and development of en do- 

 chrome must ])rocecd pari passu, when we recollect that the sili- 

 cious skeleton, once formed and consolidated, ceases to grow, 

 and therefore that the increase of the lighter contents must 

 materially diminish the specific gravity of the entire frustulc. 



Of course the development of the freshwater forms depends 

 on precisely the same laws as that of the pelagic ones ; but their 

 buoyancy is constantly obscured by entanglement amongst Algfc 

 and other bodies mixed with the water in which they reside. 

 There are two easily available modes in which the buoyancy of 

 certain freshwater species, and the powerful light-seeking ten- 

 dency of others, may be tested. The first consists in carefully 

 scooping up, during bright sunny weather, a portion of the water 

 in the immediate neighbourhood of the larger Algae; the second, 

 in watching how rapidly the mud-inhabiting species cover a 

 muddy bank (which, during stormy weather, exhibited not a 

 single frustule) with a uniform layer of glistening and satin-like 

 yellow. 



Lastly, the spaces frequently traversed by the Diatomacese 

 are such as to be quite irreconcileablc with the ordinary alter- 

 nating to-and-fro motions they exhibit ; for, even granting that 

 this kind of motion were capable of being continuously exercised 

 in any given direction, the speed achieved by it would be quite 

 disproportionate to the distances travelled over. 



Having thus far endeavoured to trace the influences which 

 determine the general limits of the Diatom as to depth, and its 

 transition from one bathymetrical range to another, it remains 

 for me to state the result of an extended series of observations, 

 conducted with a view to ascertain the precise agency whereby 

 the ordinary motile power of the free species is produced. 



Did space permit, or were it necessary after what has fallen 

 from the pen of the late Professor Smith on the point, I might 



* It is well worthy of note that the ordinary to-and-fro movements of 

 the Diatomacea; aj)pear to be carried on quite as cncrp^etically under arti- 

 ficial as under solar light. IJut artificial light does not bring them to the 

 surface as solar light does, — clearly ])roviiig that the ascent of the free- 

 floating forms from the depths is not duo to the ordinary motile power of 

 their frustules. 



