434 E. J. ALLEN. 



stances which the diatoms require, but also substances which in higher 

 concentrations are detrimental to growth, whereas in low concentrations 

 their inhibitory action is reduced or disappears. 



CHANGES IN THE COMPOSITION OF THE ARTIFICIAL 



SEA-WATER. 



A series of experiments was made to ascertain to what extent the 

 composition of the artificial sea- water could be changed without affecting 

 the growth of Thalassiosira, and it was found that, provided 4 per 

 cent of natural sea-water were added, the various constituents of the 

 artificial water might be varied to a surprising extent without in any 

 way retarding the growth. Only those results are included here which 

 were quite marked and definite. Other variations in composition were 

 tried, but an account of these is reserved until the experiments have 

 been repeated and extended. 



Varying the Amount of Magnesium Sulphate. A series of flasks was 

 set up, the basis of the culture medium in each being artificial sea- water 

 prepared according to the table on p. 424, the quantity of magnesium 

 sulphate being varied. The full amount of alkali favourable to diatom 

 growth was added (i.e. 2-6 c.c. of M.NaHCOg per litre), together with 

 the usual quantities of 20 per cent KNO3 and Miquel's solution B 

 (Na2HP04 ; CaClg ; FeClg ; HCl) and 4 per cent of natural sea- 

 water. The series contained {a) no magnesium sulphate, (b) j the normal 

 amount, (c) | the normal, (d) f normal, (e) the normal amount, i.e. 

 29-06 c.c. of M.sol. per litre, (/) 1^ times the normal and (g) 1^ times the 

 normal. All the flasks were inoculated in the same way with Thalas- 

 siosira gravida. During the first month all the flasks gave excellent 

 growths, and it was not possible to distinguish between them. At the 

 end of three months (a) and (b) had gone off more than the others, and 

 (/) and (</) were not quite up to (c), {d) and (e). A repetition of (a) to (e) 

 again gave the same result, the cultures being particularly large 

 and healthy. In speaking of this result, it must be remembered that 

 although the only sulphur present in (a) was that introduced in the 

 4 per cent of natural sea- water a considerable amount of magnesium was 

 present as magnesium chloride. 



Varying the Amount of Calcium Chloride. Another series of experiments, 

 was made in every respect similar to the last, excepting that the calcium 

 chloride in the artificial water was varied instead of the magnesium 

 sulphate, which remained normal : («) contained no calcium chloride, 



