FOOD FROM THE SEA. 393 
as a whole, a piece of the seaweed was burnt and the resulting ash 
added to the artificial solution, the diatoms did not grow. 
These two experiments together make it probable, though they do 
not completely prove the point, that it is an organic extract of the 
seaweed, and not some inorganic salt dissolved out of it, which has 
made the artificial solution a suitable medium for diatom growth. 
In other experiments it was found that if instead of adding a small 
quantity of natural sea-water from the open sea, the same quantity of 
sea-water taken from the tanks of the Aquarium were added, the resulting 
diatom cultures were larger and more vigorous. The Aquarium water seems 
therefore to contain more of the substance whose nature we are trying 
to determine than the water from the open sea. Now in the Aquarium 
animal life is more concentrated than in the sea outside, and one chief 
difference between Aquarium water and that brought in from outside 
is that the former contains more of the waste products of animal life. 
The suggestion here again is that the substance we are seeking is an 
organic substance, a substance produced by some living organism in 
this case a waste product of the living fish. 
It must, however, be one of the more stable organic substances, for 
it was found that if some of the Aquarium water was evaporated to dry- 
ness and the remaining salt heated to a comparatively low temperature, 
say, to 200° C., and then again dissolved in distilled water, the solution 
thus obtained was practically as effective as natural sea-water in pro- 
moting diatom growth. The effective substance therefore can be dried 
and heated to at least 200° C. without being destroyed. When, how- 
ever, the heating was carried further, until a dull red glow was produced, 
the substance was destroyed or so altered that it ceased to stimulate 
growth. 
Again we have the suggestion that the growth stimulant is probably 
an organic substance rather than a mineral salt. Further than this it 
has not up to the present been possible to carry the matter. In addition 
to small quantites of many inorganic salts, traces of a number of organic 
substances have been tried, amongst them being asparagin, calcium 
succinate, calcium malate, theobromine, tyrosine, urea and uric acid, 
but in all cases the results were negative. 
An observation recorded by my colleague Mr. L. R. Crawshay is of 
much interest in connection with this point. Mr. Crawshay was carrying 
out experiments in which he tried to keep alive in the Laboratory some 
Copepods belonging to the species Calanus finmarchicus, which he was 
feeding on the diatom Nitzschia. He found that in the vessels contain- 
ing the animals the diatoms grew and multiplied rapidly. If, however, 
the diatoms were put into similar vessels in which no animals were pre- 
