34 The Sea 
becomes red the burette is turned off and the percentage of 
salinity is read directly from its scale. Special apparatus and 
standard solutions for this purpose are available through most 
chemical supply houses. Also available are hydrometers for 
salinity determination. Another instrument used is an elec- 
trical conductivity apparatus which determines the amount 
of salt in a sample of sea water by measuring the amount of 
current which can pass through the solution. This latter type 
of apparatus is somewhat more expensive than the hydrome- 
ters or chemical solutions. 
CHANGES IN COMPOSITION OF THE SEA 
One of the remarkable aspects of ocean water chemistry 
is the fact that although the major constituents of sea water 
may be present in greater or lesser quantities (that is, the 
water may be more or less salty), the ratio of the constitu- 
ents among themselves is almost always the same. Another 
way of putting it is to say that although the absolute propor- 
tions of the dissolved substances in sea water vary consider- 
ably, the relative proportions are remarkably constant. 
There are some important exceptions to this rule, such as 
the proportions of such elements as silica, phosphorus, and 
combined nitrogen, which vary considerably depending upon 
locality and season. The reason for these exceptions to the 
rule is that they are necessary elements for the life processes 
of living plankton (the tiny floating animals of the sea) and 
are absorbed in large quantities from one time to another and 
at other times are redeposited. The balance of such sub- 
stances is of the utmost importance in controlling the cycles 
of life that occur in the sea, and plays a significant role as a 
primary factor in the biological environment. Much study 
