INTERACTION OF SALTS. 



49 



ARTIFICIAL SEA-WATER AND THE EFFECTS OF THE SALTS OF SEA- 

 WATER, AS A WHOLE, UPON PULSATION. 



In the experiments upon Cassiopea the solutions containing some or 

 all of the chief constituents of sea-water were made up in accordance 

 with the formula given by Dittmar (1884)*, and also according to 

 Van 't Hoff's formula (100 NaCl +2.2 KC1 + 7.8 MgCl 2 +3.8 MgSO 4 + 

 3 CaCl 2 , all of ^n concentration. 



Medusae pulsate normally in an artificial sea-water made according 

 to Van 't Hoff's formula, but pulsation is somewhat irregular in a 

 sea- water made according to Dittmar 's formula. Table 4 shows the 

 results of experiments with Dittmar's formula, and table 5 gives the 

 results obtained by using Van 't Hoff's formula. 



Tables 4 and 5 show the effects upon Cassiopea of various solutions 

 containing one or more of the constituents of sea- water. It will be 

 apparent that magnesium is the chief restrain er of pulsation, and that 

 it prevents the spontaneous contraction of disks deprived of mar- 

 ginal sense-organs and retards pulsation in perfect Medusae. When 

 magnesium is present the absence of calcium quickly stops pulsation , 

 but when magnesium is absent we may have calcium also absent and 

 the Medusae will pulsate for a considerable time. It is apparent, 

 therefore, that calcium assists the NaCl to counteract the retarding 

 influence of magnesium. This is also shown by the fact that Medusae 

 pulsate for a long time in Na + Mg + Ca, whereas all movement ceases 

 very soon in Na + Mg. 



Potassium, however, does not assist the NaCl to resist the stupefy- 

 ing influence of magnesium, for Medusae cease to pulsate almost as soon 

 in Na + Mg + K as they do in Na + Mg. Potassium serves mainly to 

 stimulate movement in connection with both calcium and sodium ; thus 

 Na + K and Na + Ca give temporary pulsations at about normal rate ; 

 whereas Na + Ca + K gives strong pulsations at fully twice the nor- 

 mal rate, but these can not be sustained indefinitely unless magnesium 

 be present to counteract the too powerful stimulating effects of the 

 Na + Ca + K. A Ringer's solution is only a powerful stimulant, and 

 can not sustain pulsation indefinitely unless tempered by magnesium. 

 Potassium has little power to revive pulsation, whereas calcium pos- 

 sesses this power to a marked degree ; thus, when pulsations have 

 ceased in NaCl they can always be revived by calcium, but at best 

 only a very few isolated contractions can be revived by potassium in 

 the amount and proportion found in sea -water. 



^Reports of voyage of H. M. S. Challenger, Chemistry, vol. I, p. 204. 



