CULTURE OF PLANKTON DIATOM THALASSIOSIRA GRAVIDA CLEVE. 425 



quisite number of cubic centimetres of M solutions of the other salts 

 (KCl 10-28, CaCl2 10-86, etc.) and then make the whole up to exactly 

 1 litre by adding double-distilled water. 



Water prepared according to the figures given in this table was titrated 

 for me by Mr. Matthews against the standard water suppHed by the 

 International Council, and was found to have a salinity of 35 per 

 thousand. 



Alkalinity. — The alkalinity has also been compared with that of sea- 

 water from outside the Plymouth Breakwater by Sorensen's method, and 

 was found to be very close to it, the artificial water being slightly less 

 alkaline. It was found experimentally that better growths of diatoms 

 were obtained when the alkalinity was increased somewhat,* the best 

 result being obtained when an extra 1-33 c.c. of Msol. NaHCOg per litre 

 was added, making a total of 2-6 c.c. of the molecular solution of this salt. 



DIATOM CULTURES IN ARTIFICIAL SEA-WATER. 



As was to be expected, it is not possible to obtain cultures of diatoms 

 in the artificial sea-water prepared as described in the last section as it 

 stands. The water must be first treated with nutritive solutions, and for 

 this purpose the modifications of Miquel's solutions described in our 

 former paperf have been used. Two solutions are employed as follows : — 



Solution A. 



Potassium nitrate 20-2 orm. ^ ^_, rnvr^ 



T^- -11 n . . =2M KNOo, 



Distilled water to 100 c.c. ) 



Solution B. 



Sodium phosphate (Na2HP04l2HoO) 4 grm. 

 Calcium chloride (CaCUGHaO) 4 ,, 



Ferric chloride (melted) 2 c.c. 



Hydrochloric acid (pure, concentrated) 2 c.c. 

 Distilled water 80 c.c. 



To each 1000 c.c. of artificial water add 2 c.c. solution A and 1 c.c. solution 

 B. SteriHze by bringing to the boil. When cool decant| of! the clear 



* Ct". Allen and Nelson, loc. cit., p. 452 [Q.J.M.S., p. 401]. The figure here given 

 was derived from later experiments. 



t Allen and Nelson, loc. cit., p. 4-28 [Q.J.M.S., p. 370]. For details as to the pre- 

 paration of Solution B that paper or Miquel's original account should be studied. 



J Filter papers should not be used to filter off the precipitate. They appear to contain 

 some substance which inhibits the growth of the cultures. The cultures were made in 

 wide-mouthed spherical glass (iasks covered with glass capsules. Cotton wool i^Iugs were 

 not used, as these were found to be injurious to the growth of the diatoms. 



