12 Dr. G. C. Wallich on the Diatomacca. 



the silicious portion of the frustule must at once be set aside. 

 Professor Bailey's observations with reference to the behaviour 

 of the Diatom when treated by hydrofluoric acid are conclusive 

 on this point. The external silicious framework is consumed. 

 The internal cellulose cell-membrane remains intact, leaving no 

 trace of any delicate network, such as we should certainly find 

 were it associated with the substance of the silicious portions. 

 On the other hand, the substance of the silicious valves (after 

 being subjected to hydrochloric and nitric acids) in no case 

 exhibits evidence of permeation by other matter. 



The frustules of certain species, it is true, are more readily 

 broken up under the action of acid; but under no circum- 

 stances can nitro-muriatic acid destroy a film of silica, however 

 delicate. 



Forms accidentally removed from their natural habitats, and 

 thereby placed under conditions for which they are unfitted, 

 naturally enough, present examples of imperfect deposition of 

 silex, as to quantity, but not as to purity. Such a state occurs 

 in freshwater forms exposed to brackish water, or vice versa. 

 But these are exceptional cases, and cannot be allowed to weigh 

 in our estimate of normally developed forms. As regards the 

 Diatoms more particularly noticed as falling under the modified 

 silicious examples, it is only necessary to state that specimens 

 subjected to boiling, for many hours, in the most concentrated 

 acids, in no single instance presented appearances which could 

 be interpreted into an obliteration of the silicious envelope. 



The true significance, I would suggest, of the delicate nature 

 of the silieious element in the Diatomacea3 under review consists 

 in a simple adaptation of the means to the end in the lighten- 

 ing of the mineral framework as far as is compatible with the 

 requisite strength, and its being made to enclose the greatest 

 possible space, in order to admit of that luxuriant development 

 of the endochrome and protoplasmic contents, in virtue of which 

 the specific gravity of the frustule is diminished and its buoyancy 

 secured. 



Light and pressure seem to be the main causes that im- 

 pose limits to the wanderings of these organisms. It has been 

 shown that their buoyancy must vary with the intensity of 

 the conditions necessary to their development; and we are 

 justified in concluding, moreover, that there are periods in 

 the history of the structure at which their development and re- 

 productive phenomena proceed most vigorously. Their bathy- 

 metrical position must therefore also be a fluctuating one ; and 

 we can thus satisfactorily account for the seemingly capricious 

 manner in which they approach, or disappear from, the surface 

 of their element. 



