210 Proceedings of Royal Society of Edinburgh. [sess. 
with care, and I warn collectors against removing spoonfuls of 
diatoms direct from the tow-net to the tube. It is better to float 
the whole capture in a glass jar, to permit the whole to settle for 
an hour or so, and then to collect from the bottom by means of a 
dipping tube. This method gives, without special calculation, the 
right proportions of diatoms and sea- water to be added to the half- 
filled tube of chromic acid solution. With such an outfit of tubes 
secured in a rack-work box, and by clamping the microscope down 
to a bench, it is possible to continue work with all powers but the 
immersion lenses even in moderately bad weather. The use of such 
high objectives as nre now commonly immersed is too fatiguing to 
the eyes while the body is swaying in different time to the vessel, 
from the difficulty of closely accommodating the eye to the ocular 
for specially minute study requiring an uninterrupted stare into 
the field. 
During the first months of the year, as Dr John Murray has 
recorded, there is in all our seas an extraordinary prevalence of 
diatom life, and I found this to be specially true of the west coast 
and Clyde sea area. It is only necessary to put down the net for 
a few minutes to obtain at this season a dense mass of diatoms 
which nearly half fills the net with a yellow-brown scum of the con- 
sistency of soft soap. Sir Joseph Hooker (on Sir James Ross’s 
Antarctic Expedition) and the naturalists of the “Challenger” 
Expedition have made us familiar with the extraordinary abun- 
dance of diatoms in the Southern Ocean. South of latitude 50° 
the tow-nets of the “ Challenger ” Expedition were sometimes so 
filled with diatom scum “that large quantities could be dried by 
heating over a stove, when a whitish felt-like mass was obtained.” 
It is not necessary to visit the Southern Ocean to see this impressive 
manifestation of diatom life in the sea. I dried similar masses over 
the boilers of the “ Garland ” in the Clyde sea area last April, and 
the prevalent form was in all cases Skeletonema costatum with a 
slight mixture of Coscinodiscus concinnus , Chcetoceros borealis, and 
other forms. This vast occurrence of diatoms is a seasonal 
phenomenon, and they become scarcer in summer (except here and 
there in particular shoals), and are largely replaced by species of 
Ceratium — notably C. Tripos, C. fuseum, and C. Fusus. For 
example, the prevalent Skeletonema of April had disappeared in 
