FOOD FROM THE SEA. 389 
basis for the culture medium it is not necessary to add an infusion of 
organic matter. 
The actual quantities of inorganic salts added to the sea-water are 
really very small, those employed in our later experiments being in 
100,000 parts of sea-water :— 
40 parts of Potassium nitrate. 
4 ,,,, Sodium phosphate. 
4, - ,, Calcium: chloride: 
2 9.4 460 Hemietehlonride: 
When the culture solution has been prepared it is first boiled in order 
to kill off any organisms it may contain, and a precipitate which forms 
is allowed to settle. The clear water is then poured off into small flasks, 
in which the experimental cultures are made, a second boiling being 
carried out before the flask is inoculated with the diatom culture. The 
diatoms grow best at a temperature of about 60° F., in a good north light. 
They must not be exposed to direct sunlight, as in the small flasks used 
this is found to kill them. 
The culture solution just described has sea-water for its basis. Now 
sea-water is a very complex solution containing both inorganic and 
organic substances, and although it is true that the relative proportions 
of the predominating salts are remarkably constant everywhere in the 
sea, there are present in it also, often in very minute quantities, many 
other important substances which are subject to considerable variation 
from place to place and from time to time. These varying substances are 
in many cases just those which are of special importance to the living 
plant. They occur often in such minute traces that it is practically 
impossible to measure accurately by means of chemical analysis the 
quantities in which they are present. 
In order, therefore, to study the effect on the growth of the diatoms 
of very small quantities of various substances a different method of pro- 
cedure was adopted. Instead of using natural sea-water as the basis 
for the culture solutions, an artificial sea-water was built up by dissolv- 
ing in pure distilled water the purest chemicals that could be obtained. 
The distilled water was made specially pure by distilling it a second 
time in all-glass apparatus, so that it did not come in contact with any 
metal such as tin or copper. Further, when being distilled for the second 
time, the water was boiled with bichromate of potash and sulphuric 
acid in order to destroy as far as possible any volatile organic matter 
what it might contain. By taking the right proportions of the pure 
chemicals and dissolving them in this doubly distilled water an artificial 
