200 SCIENCE PROGRESS 



the former area was 7*2° C. to n"3° C, and that of the latter 

 22*i° C. to 29/4 C. The warmer seas were, therefore, richer in 

 phosphoric acid than were the colder ones — a distribution 

 which, assuming Schmidt's results to be reliable, is not easily 

 explained. The difference in the temperature is not by itself 

 sufficient, though it is of course possible that there is more 

 carbonic acid in the colder seas than in the warmer ; and since 

 phosphoric acid is more soluble in water containing an excess 

 of this gas, it may be that the difference is thus indirectly due 

 to the difference in temperature. Phosphoric acid is also more 

 soluble in water containing a high proportion of dissolved salt 

 than in weakly saline water ; but, again, it is not likely that 

 the differences in the salinity of the sea are sufficiently great 

 to account for the variation in the contents in calcium phosphate. 



It is, however, probable that Schmidt's analyses were de- 

 fective. Quite lately Raben made a number of determinations 

 of the amount of phosphoric acid in the water of the Baltic Sea, 

 and found that the amount was much less than was supposed 

 from the results of the earlier analyses. Raben's results are 

 not yet published, but Brandt quotes some of them, 1 and it 

 would appear that the amount of P 2 5 in these waters varied 

 from 0*14 to 0*25 parts per million in February and May, and 

 might amount to about 1*4 per million in the autumn. 



Silica is the third important foodstuff in the sea so far as 

 the plankton is concerned, and the determination of the pro- 

 portion in which this substance occurs in the sea is of the 

 utmost significance for an understanding of the conditions 

 under which the production of living matter is carried on. 

 Most of the older analyses are of little value, and the only 

 results which are likely to be accurate are those of the analyses 

 made by Raben in 1902-4. Silicic acid can only be determined 

 by converting it into an insoluble form, and when minute quan- 

 tities of this substance are associated with large quantities of 

 soluble salts the determination is a matter of great difficulty. 

 Raben evaporated 3 litres of the water sample to dryness in 

 a platinum basin, after most careful filtration, so as to remove 

 the diatoms which would obviously give high results if they 

 were allowed to remain in the sample examined. The residue 

 in the basin was then drenched with hydrochloric acid and then 



1 Brandt, Rapports et Proces-Verbaux, Conseil Permanent International, 

 Exploration de la Mer, vol. ill- 



