272 MESSRS. C. SHEARER, W. DE MORGAN, AND H. M. FUCHS 



difficulty we have encountered with the use of this water arises from the fact that 

 its purity and higli allvalinity render it a medium in which bacteria and infusoria 

 multiply with great rapidity. 



The true alkalinity of the "outside" water, that is the concentration of the 

 hydrogen ion, expi'essed in gramme-equivalents per litre, has been determined at 

 Plymouth by D. J. Matthews by means of the colorimetric method of Sorensen. 

 According to Sorensen's (82) determination the dissociation constant of pure water 

 is I0~'^" at 18° C, and a litre of pure water at this temperature would therefore 

 contain 10"'"' gramme-equivalents of hydrogen ions, and the same number of hydroxyl 

 ions. He expresses for convenience the acidity of a solution by the symbol Pji, 

 which is the numerical value of the exponent of the concentration of the hydrogen 

 ion, with the sign changed. Thus for pure neutral water Pg, owing to the change 

 of sign, the higher the value of P^ the lower the acidity, or the greater the alkalinity. 

 See Palitzsch (72). 



SoRENSEN carries out his test by adding to 10 c.c. of the solution in a test-tube a 

 certain number of drops of various indicators, and comparing the resultant colour 

 with that of a series of tubes for which Ph has been determined by electrical 

 measurements. For sea-water he uses : 



(a) Phenolphthalein — 0'5 grm. in O'o litre of weak alcohol, eight drops for 



a test. 



(b) a-Naphtholphthalein — 0"2 grm. in 0"5 litre of weak alcohol, six drops for 



a test. 



The above indicators and standard solutions of hydrochloric acid and sodium 

 borate cover the range met with in the sea. Aquarium water may reach, however, 

 the lower limit of Ph = 7 "4. 



In the late summer and autumn of 1911 the "outside" water at Plymouth gave 

 a distinct red colour with phenolphthalein, and Py was consequently above 8"0 ; 

 generally 8-15 to 8-25. When the tests were resumed in the spring of 1912, the 

 water gave no colour with phenolphthalein, and tests with a-naphtholphthalein 

 showed about Pii=7"9, P,i gradually increasing during the summer till the con- 

 ditions were much the same as in the previous year. 



According to Allen and Nelson (3), the average salinity of " outside " water is about 

 S = 34'5-35"5 per cent., and the temperature range for the year is from 8° to 16° C 



(2) By " Berkefeld " water we mean ordinary sea-water taken from the laboratory 

 tanks, treated with animal charcoal, passed through a Berkefeld filter, and then 

 stored in sterilized glass carboys. We have made use at Plymouth of the automatic 

 apparatus invented by Allen and Nelson (3) for the preparation of this water. 

 This has been described by them in detail in their paper. We have used this 

 water for the larval stage of growth in our experiments, as the process by which it 

 is prepared frees it from bacteria and putrefactive products. In its use there is not 



