24 DIATOMS. 



used as flour by the Lapps, Indians, and Chinese, in times of 

 dearth. Bergh Mehl is an indefinite term, and applied to all 

 diatomaceous deposits of the same character as those in Norway 

 and Sweden. These two are exceedingly interesting, and one is 

 composed entirely of diatoms, as may be seen under the micro- 

 scope. 



By this short list, we see that many different forms are to be 

 found in certain localities, and it occurs to me to enquire whether 

 there are any conditions of soil or water to which these peculiari- 

 ties are owing ; for instance, why do we get polygonal forms in 

 one place and a rounded or semi-circular form in another ? 



Circular forms are more abundant in sea-water. The Rev. 

 W. L. Smith, in his " Synopsis," says that " The discoidal forms 

 of diatoms constitute about 30 per cent, of the total number of 

 genera — the polygonal forms about 10 per cent. — and the remain- 

 der have more or less of a Navicula contour." Most of the fresh- 

 water genera are represented in brackish and in sea water, and by 

 far the larger proportion of discoidal and polygonal forms are 

 marine. The presence of Silica in water has much to do with 

 the robustness of the diatoms. I fancy that I have noticed, in 

 some instances, that the harder the water the more elongated in 

 shape are the diatoms, but of this I am not yet satisfied. 



But all of these organisms are worthy of notice. Being so 

 numerous, no doubt they play some important part in Nature's 

 economy, as w^hen living they assimilate and appropriate the 

 soluble constituents of the water, especially Silica. They give off, 

 also, as may be seen in the living specimens, a large quantity of 

 Oxygen, that must not only impregnate the water, but as it is one 

 of Nature's purifiers, must assist likewise in keeping it pure and 

 wholesome by the oxygenation of its constituents. This suggests 

 the thought, " Would not the presence of so much Silica make it 

 an excellent filtering medium ? "' We know how well the Silica- 

 filters do their work, and if diatomaceous earth could be obtained 

 in sufficient quantities, it would no doubt make a good filtering 

 medium by being first burnt, and then mixed with the usual 

 proportion of carbon. 



I hope, by these few and unconnected remarks, that 

 some will be led to look more after, and investigate these 

 beautiful and wonderful structures. If we just think that 

 one of these little objects requires a power of 400 or 500 

 diameters at least, to show anything like its structure, and that it 

 exists in millions and millions in our rivers, lakes, and streams, 

 and is playing such an important part in the world's history, we 

 shall then see the value of studying them, not simply straining 

 our eyes to resolve dots, and holding long arguments as to 



