256 H. S. H. WARDLAW, 
compensatory dialysis, the solution in which the amount 
of a certain constituent in a diffusible condition is to be 
determined is dialysed, not against water, but against a 
second solution so made up as to contain in a diffusible 
condition all the constituents of the second solution except 
the one the amount of which is to be determined in such 
concentrations that only the constituent under observation 
will diffuse, all the other constituents being balanced or 
compensated by equal concentrations outside the membrane 
through which dialysis takes place. Apart from the fact 
that one would require to know a great deal about the 
solution being examined before such an outer liquid for the 
dialyser could be prepared, this balancing of all of the con- 
stituents of a solution but one does not at all mean that 
the natural equilibria will remain undisturbed. Returning 
to the analogy with the equilibrium between a salt and its 
ions, it is known that any alteration in the concentration 
of any one of the ions will bring about a re-adjustment of 
concentrations of the other substances present necessary 
to reach a state of equilibrium under the changed con- 
ditions. Thus, although in special cases it may be possible 
to prepare these compensating solutions, as Rona and 
Michaelis have shown, this method does not seem to possess 
a wide range of applicability. 
These authors, however, have employed another method 
for the examination of the diffusible substances of milk 
which reduces toa minimum all displacement of equilibria. 
This method is simply an extension of that of Moore and 
Bigland. The liquid under examination is allowed to dia- 
lyse against a known volume of water, but in this case the 
volume of the water is made very small in comparison with 
that of the milk (25 cc. of water to 1000 cc. of milk). The 
milk is thus only slightly diluted and a much truer estimate 
of the diffusible substances may be formed. By this method, 
and by the method of compensatory dialysis, Rona and 
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