2C0 
F. KAMSOM—DIATOMS: 
is also usually present a kind of oil existing in minute globules in 
various parts of the frustule. In the ‘ Micrographic Dictionary ’ 
it is suggested that these globules represent the starch-grains 
frequently found in confervoids. On drying and heating diatoms 
a peculiar odour is emitted which has been ascribed to this oil. 
The remainder of the interior space is occupied by water, which is 
continually passing through the minute apertures at the extremities 
of the frustule. A constant current is thus maintained, and, as the 
water charged with carbonic acid passes through, the process of 
assimilation occurs, the carbon being retained and the oxygen 
liberated. This oxygen, being less soluble than the carbonic acid, 
may frequently be seen rising in bubbles to the surface of the water 
wherever there is any large aggregation of diatoms below. This 
power of liberating oxygen is one of the strongest reasons for 
placing the order in the Yegetable Kingdom. Several of the mineral 
materials dissolved in the water are also retained for use, including 
silica that is required for the construction of the valves, and nitrogen 
compounds for the formation of protoplasm. 
The markings on the valves of diatoms have recently received 
a large amount of careful examination, and many points respecting 
their structure have been explained. A summary of these, together 
with the results of a large amount of valuable work by Dr. Flogel, 
has recently appeared in the ‘ Journal of the Royal Microscopical 
Society.’ Similar investigations have also been recorded by J. D. 
Cox in the ‘ American Monthly Microscopical Journal.’ The 
inferences drawn from these researches indicate that the markings 
of these shells are due to the various forms of depressions and 
inequalities in the siliceous envelope, and that no single theory is 
applicable to all the appearances. Owing, however, to the extreme 
minuteness of these markings, great difficulty is experienced in the 
investigation of them. Some interesting phenomena, which might 
at first be supposed to throw some light on the construction of the 
diatom-valve, were investigated by Schultze with this object. On 
adding sulphuric acid to a mixture of powdered fluor-spar and 
sand, a colourless gas (silicon-tetrafluoride) is evolved, which on 
coming in contact with aqueous vapour deposits silica in a very 
finely-divided condition. On examining this deposit under the 
microscope it is found to consist of thin-walled vesicles of various 
forms and sizes. The exterior of many of these exhibit markings 
very similar to those which are seen on the valves of many common 
diatoms, such as Pleurosigma and Coscinodiscus. The results of 
Schultze’s investigations tended however to show that the differ¬ 
ences were greater than at first appeared, and he concludes that 
the sculpturings of diatom valves and siliceous pellicles are in 
reality due to wholly different conditions. 
The habitat of diatoms forms of itself a subject of much interest. 
Roughly, they may be divided into three classes, according to the 
water which they inhabit. Firstly there are the fresh-water 
species, including all those which exist in rivers, ponds, or ditches, 
and indeed in any situation to which the sea has no access; secondly 
