226 
MARINE AND ESTUARINE DIATOMS. 
may be placed in or over the material sought, which will, by 
suction when the finger is withdrawn, pass up into the bottle, to 
be there tightly corked. Or a quantity of pond scum may be 
directly dipped up by the hand or a cup and introduced into the 
bottle. If this is to be kept for any considerable length of time 
it is well to add a few drops of a weak solution of formaline to 
prevent putrefaction. Mud from river, pond or lake bottoms 
may be obtained when the waters are not too deep, by cups or 
scoops attached to poles of suitable length, while submerged 
objects of almost all kinds are covered with a scum, often richly 
diatomaceous, which may be readily scraped off with the edge 
of a knife. For plankton collections, as already stated, very fine 
silken nets are required. The collection having been made, it 
may be necessary to remove impurities. If these consist of sand 
or mud, a separation may be made by mere decantation, the 
Diatoms being usually much lighter than ordinary mineral 
matters. If the collection is from sea water, the salt must be 
gotten rid of, as otherwise it will crystallize upon the glass slides, 
but the removal is easy as the result of the addition of fresh 
water and decantation until all traces of salt is removed. Finally, 
if there be much organic matter either in or with the Diatoms, 
it will be well to place the material in a test tube and boil for 
some minutes with strong nitric acid, subsequently removing 
this as before by dilution and decantation. 
A good microscope is required for all work with Diatoms 
and high powers are needed for satisfactory results. A one- 
quarter inch or a one-sixth inch objective, such as made by 
Leitz, Zeiss, or Bausch and Lomb, answers well for ordinary 
observations, but for the smaller and finer forms a one-twelfth 
oil immersion will be needed. Some means of exact measure- 
ment, in other words some form of micrometer, will also be 
required for specific determinations. , 
The material having been cleaned, as above described, is 
ready for mounting. This is done on ground glass slides which 
may be obtained, with thin glass covers, from dealers in 
microscopic supplies. (Lyman & Co., Montreal). A small 
