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Proceedings of Boyal Society of Edinburgh. [sess. 
quantity of silicate of alumina or clay present in sea-water, even at 
very great distances from land, and in the saltest and warmest 
waters. The above waters were taken from the surface ; hut, by a 
carefully-collected series of waters from different depths, it might 
be shown that the deeper and colder waters contained a greater 
proportion of this fine clayey matter than the surface ones, and 
it is at once apparent that waters taken near shore will contain 
more than those from far out at sea. 
Bearing in mind the above facts, it is interesting to recall what was 
stated above as to distribution of siliceous organisms in the ocean, they 
being more abundant in shore waters or in waters of a low salinity 
and in cold waters — as, for instance, Diatoms in brackish waters and 
in those of the cold Southern and Polar Oceans, and Eadiolaria in 
polar waters and in the West Pacific and Eastern Indian Oceans, 
where there is a relatively low salinity, as well as in deep inter- 
mediate waters where there is a low temperature. This would seem 
to indicate that in the ocean siliceous organisms are more abundant 
where there is most clayey matter in suspension in the sea-water. 
With the view of gaining some information as to the conditions 
under which silica might be secreted by organisms, we instituted a 
number of experiments with Diatoms and other silica-secreting plants. 
A culture solution, representing the mineral food of plants according 
to Sachs’ formula, was prepared, consisting of — 
Distilled water, . . 2000 grms. 
Chloride of sodium, 1 ,, 
Nitrate of potash, . 2 ,, 
Sulphate of lime, . 1 ,, 
Sulphate of magnesia, . 1 grm, 
Phosphate of lime, . 1 ,, 
Ferric chloride, . . 1 ,, 
(A) Into a portion of this solution a minute patch of Diatoms 
( Navicula ) was placed (in August 1890) with a small quantity of 
silicic acid in the form of jelly. In the course of seventeen days 
they grew most vigorously, the Diatoms increasing in great numbers 
—possessing the characteristic yellow-green colour of chlorophyll, 
giving off oxygen abundantly in sunlight, and moving about with 
the peculiar motion of these organisms. Prom this patch of Diatoms 
we obtained the material for the following experiments. 
(B) A small quantity of living Diatoms from (A) was carefully 
washed so as to remove all traces of silicic acid or soluble silicates, 
