CH. X] THE CONDITIONS OF LIFE IN THE SEA 213 



frustules, of diatoms ; the skeletons of radiolaria ; and the spicules 

 of some sponges. Lime, which is usually present in solution 

 as soluble calcium bicarbonate, is required for the formation of 

 the bones of fishes ; the shells of molluscs, such as the mussel or 

 oyster; the exo-skeletons of Crustacea; the hard parts of corals, 

 echinoderras, polyzoa, some sponges, and some algae ; and the 

 skeletons of many protozoa. 



(6) Certain other mineral salts, such as the chlorides, 

 sulphates, &c., of sodium, potassium, magnesium, and iron. Only 

 small quantities of these are required. 



(7) Oxygen. This is hardly to be regarded as a food-stuff", 

 but it is necessary for the respiration of both animals and plants. 



We may now consider the proportions in which these various 

 food-stuffs are present in the waters of the sea. 



Nitrogen -com pounds. These are present in the sea in 

 excessively small proportion, so small that their determination 

 has always been a matter of great difficulty. Many series of 

 analyses have been made from time to time. The Challenger 

 Expedition, which did pioneer work in this, as in so many other 

 directions, collected samples of water from various parts of the 

 sea, and these were analysed by Murray and Irvine some years 

 after the return of the ship. But so rapid has been the progress of 

 oceanographical investigation that we speak of the methods of the 

 last dozen years or so as " modern " in comparison with those of 

 thirty years ago. Our knowledge of the distribution of nitrogen 

 compounds in the sea is due to Natterer^ Raben-, and Piitter^ 

 Natterer investigated the Mediterranean, the Sea of Marmora, and 

 the Red Sea, both at the surface, and at various depths from this 

 to the bottom. He used the best methods of estimation available 

 at the time, and the samples of water collected were examined on 

 board the ship immediately after being taken — an important pre- 



1 " Chemische Untersuch. in ostl. Mittelmeer.," Ber. Cornm. Erforsch. ostl. 

 Mittelmeeres. In Denksch. Akad. IViss. Wien, Bde. lix. lx. lxi. lxii. lxv. 1892- 

 1898. 



2 Wis8. Meeresunt. Kiel Komm. Bd. viii. Abtli. Kiel, pp. 83, 277, 1905. See 

 also Brandt, Bappts. et Proc.-Verb., Cons. Perm. Internat. Explor. Mer. vol. iii. 

 Appdx. D, p. 7, 1905. 



^ " Studien zur vergleichenden Physiologie des Stoft'wechsel," Abhandl. konigl. 

 Ges. Wissensch. Gottingen, Math.-Phys. Klasse, N. F., Bd. vi. No. 1, 1908. 



