THEIR NATURE AND HABITS. 
205 
they exist in a condition almost the same as when originally 
formed, is perhaps open to question. Occasional specimens are met 
with in recent geological strata. One of the most interesting 
discoveries of this kind was made by Mr. W. H. Shrubsole, who 
found them to exist in the London Clay. Most of the specimens, some 
of which remain unbroken, have a deposit of iron pyrites within the 
frustule, which, however, can be removed with dilute acid. A few 
new species have been detected, but the great majority are identical 
with those living at the present time. Deposits of almost pure diatoms 
exist in the beds of several small lakes in North Wales, Scotland, 
and Ireland. In Norway and Sweden a remarkable use is made of 
deposits of this description, which are termed berg-mehl, or mountain- 
flour. In bad times it is the custom of the natives to mix this 
material with their dough in making bread. It has been suggested 
that any nourishment derived from it is due to the presence of some 
organic matter, but a more probable explanation of its use is that, 
by simply enlarging the bulk of bread eaten, it increases its value 
as nutriment. One of the best known and most remarkable of diato- 
maceous deposits exists in Yirginia, where the town of Richmond is 
actually built upon a stratum eighteen feet in thickness, consisting 
almost entirely of valves of diatoms of past ages. Another interest¬ 
ing source is guano, where we find that these indestructible shells 
have passed uninjured through the stomachs of sea-birds. They 
are often present in such abundance that some of the remarkable 
value of this manure in promoting the growth of cereal crops may 
be due to the silica thus contained within it, and which is so 
essential for the food of all grasses. Some new species were also 
discovered in the stomachs of tinned Japanese oysters exhibited at 
the Fisheries Exhibition in 1883. 
Diatomaceous earth is much used for polishing hard surfaces, and 
a well-known variety found in Austria is of special value for 
this purpose. Owing to the extreme minuteness of the valves a 
similar deposit is used in the manufacture of dynamite, as affording 
silica in a finely-divided condition. Besides these commercial uses, 
a most valuable service has been rendered by certain diatoms 
in providing tests for the quality of object-glasses. At one time the 
power of resolving certain of the more delicate markings on various 
species of Pleurosigma was regarded as an accurate test for the 
defining power of the lens, but it has now been shown that this 
power depends upon the magnitude of the angular aperture of the ob¬ 
jective. Much indirect good has nevertheless resulted from the 
prominence to which diatoms thus attained, by inducing increased 
efforts on the part of makers to bring their glasses to a higher 
state of perfection. f 
A rather novel use was made of diatoms a few years ago by Dr. 
Bossey, resulting in a paper entitled “Thames Mud in relation to 
Sanitary Science.” It is well known that large deposits of mud 
are continually being formed in the Thames in the neighbourhood 
of Chelsea and elsewhere, and it became an important matter to 
determine whether these accumulations had any connection with 
