THE LAW OF THE MINIMUM IN THE SEA 201 



again evaporated to dryness, and this procedure was repeated 

 three times. Then the solid mass obtained was broken up and 

 acidified with hydrochloric acid, dissolved in water, filtered, 

 and the insoluble residue ignited in a platinum crucible. The 

 insoluble residue was weighed and regarded as silicic acid. 

 But if it had any trace of colour it was wetted with sulphuric 

 acid and hydrofluoric acid was added, the mixture heated, when 

 the silica was expelled as silicon tetrafluoride. Ammonium 

 carbonate was then added so as to get rid of the traces of 

 sulphuric acid on further ignition, and the residue again 

 weighed and the weight deducted from the first weight. In 

 this way Raben obtained what we must regard as the most 

 probable values for the amounts of silica contained in the sea. 

 These results are : x 



Milligrammes of SiO.. present in the water of the Baltic per litre (equal to parts 

 per million) : 



1902, August (4 

 November (3 



1903, February (2 

 May (1 

 August (1 



Thus the amount of silicic acid in the sea is extremely small, 

 but it should be noticed that there is a well-marked seasonal 

 variation which is exhibited in each of the three years studied. 



Now it is evident that there are at least three things (com- 

 pounds of nitrogen, silicic acid, and phosphoric acid) in the 

 waters of the sea which are present in extremely small quantity, 

 and each of which is essential for the growth of the plankton. 

 If one of these is present in greatly reduced proportion, then it 

 is evident that the production of organic matter by the plants 

 of the plankton, and so by all other organisms in the sea, must 

 be correspondingly reduced. Which of these is it that governs 

 the production? Reasoning only from a knowledge of the 

 older analyses of Schmidt, Brandt came to the conclusion that it 

 was the inorganic compounds of nitrogen that were so instru- 

 mental. Taking the case of phosphoric acid first of all, he had 

 no difficulty in showing that more phosphoric acid was contained 

 in the sea than nitrogen compounds, but that much more of the 

 latter were required by the organisms of the plankton. The 



1 Raben, loc. cit. Anhang, Quantitative Bestimmung der im Meerwasser 

 geloste?i Kieselsaure. 



