ARTIFICIAL CUI/i'UKE OF MAKINE PLANKTON OKGANISMS. 897 



Some of onr eai'lier experiments seemed to show that 

 " alkalinity " was a factor of considerable importance for the 

 successful growth of cultures of plankton diatoms ; so an 

 attempt was juade to analyse the various samples of water 

 both before and after treatment as culture media. The 

 method adopted was a modification of that used by Tornoe 

 and Dittmac. Solutions of NaOH and H3SO4 of strength 

 N/jQ, by intention, were made up and stored in five-litre 

 " aspirator " bottles. Two accurately graduated burettes 

 standing side by side were connected to these by tubes, so 

 that they could be readily filled by gravity. All air inlets 

 to burettes and stock bottles were fitted with tubes of soda 

 lime. A standard solution of Na^CO^ of exactly known 

 alkalinity, approximately that of average sea-water (40*00 

 mgrm. OH %o)j was prepared by diluting down from a N/n,, 

 solution, all operations being performed by weighing. These 

 standards were stored in stoppered bottles of the fairly 

 insoluble dark green glass, but those that had been kept for 

 any length of time were riot trusted, fresh standards being- 

 prepared. On analysis these standards agreed with one 

 another to well within '1 mgrm. OH °/qq. The water used for 

 diluting the standards was distilled water from the laboratory 

 still, re-distilled in all-glass apparatus with potassium bichro- 

 mate and sulphuiic acid. 



■ When carrying out an analysis, equal volumes (about 100 

 c.c.) of sample and standard were measured out into Jena 

 glass Erlenmeyer flasks with a Knudsen automatic pipette. 

 The specific gravity of each was determined by weighing in a 

 25 c.c. pyknonieter. Sample and standard were then titrated 

 by running in acid from the burette and back titrating with 

 alkali, using a 1 per cent, alcoholic solution of aurine as an 

 indicator and keeping the liquid boiling. The acid to alkali 

 equivalent was determined by titrating a pipetteful of double 

 distilled water in the same manner. The mean of at least four 

 readings ^vas always used. Let N and n be number of burette 

 divisions of alkali equivalent to standard and sample respec- 

 tively, and D and d their density at the time of pipetting out. 



VOL. 55, PART 2. NEW SERIES. 26 



