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ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 141 
presence of a small concentration of potassium salt is necessary in 
addition to that of sodium and calcium. We may perhaps be satisfied 
with the statement that potassium is required on account of particular 
chemical properties. This appears to be the view taken by Loeb and 
Cattell (1915), but, as we have no idea as to what these properties 
are, very little isgained. Some recent work by Zwaardemaker (1918) 
is of importance with respect to the elncidation of the problem. 
This observer noticed that the elements which Ringer found to be 
able to replace potassium, namely, rubidium and caesium, are, like 
potassium, weakly radio-active and he proceeded to test other sub- 
stances more powerfully radio active, such as radium itself, emanation, 
uranium, and thorium. It was found that equally radio-active con- 
centrations of all the elements named were equal in their capacity of 
replacing potassium in a solution adequate to maintain normal cell 
life. 
Sodium salts are chosen to make up the correct osmotic pressure 
of these fluids because they are the least toxic salts. But Clark (1913, 
p. 77) found that a better solution was obtained if a part of the 
sodium chloride was replaced by its osmotic equivalent in cane-sugar. 
Some very interesting conclusions have been drawn by Macallum 
(1904) from the fact that the salts present in sea water form an appro- 
priately balanced mixture for the cells of the higher vertebrates, 
when the sea water is diluted to the correct osmotic concentration. 
It seems evident that the electrolyte composition of the blood of the 
present land vertebrate is that of the ocean at the close of the 
Cambrian period, when their ancestors left the water and took to the 
land. The Cambrian period was an extremely long one and the 
colloidal systems of the cell were developed in adjustment to this 
balanced mixture of salts. But at the same time, it is a remarkable 
fact that the particular mixture arising from the dissolving of con- 
stituents of the earth’s surface should be that of a “ balanced” 
solution, not only for protoplasm, but also for emulsions of oil and 
water. 
There is another interesting way in which the relationship of 
colloids to electrolytes meets us in physiological phenomena, namely, 
the mechanism of muscular contraction. Blix (1891) and A, V. Hill 
(1913) have shown conclusively that the tension developed is pro- 
portional to the area of certain surfaces arranged longitudinally in 
the muscle and is not a volume effect. Fitzgerald (18/78) and Bern- 
stein (1901) had already suggested surface tension at the contact 
between the fibrille and sarcoplasm as the mode of production of the 
muscular force, and there are other facts which confirm the view 
that surface phenomena form a component part of the complex of 
events. Surface tension has the peculiarity of possessing a negative 
temperature coefficient, doubtless connected with the absence of any 
boundary surface at the critical temperature. The contractile stress 
produced by muscle has a negative temperature coefficient (Bernstein, 
1908), as also has the heat produced in the initial stage. Lactic acid 
‘is one of the chemical products of the muscular process, and Haber 
and Klemensiewicz (1909) put forward the hypothesis that the acid 
alters the electrical forces at the boundary between the fibrilla and 
the sarcoplasmic liquid. This again involves a change of surface 
