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how great the [C,0,"] is while only the merest sign of turbidity 
due to CaC,O, can be detected. In general it can be done in the 
following way: 
A series of small tubes, each containing 1 e.c. of a known CaCl, 
solution, was taken, and to the tubes in succession quantities of 
oxalate solution increasing gradually with each new tube were added 
with a capillary pipet divided into tenthousandths of c.c. The tubes 
were then left to themselves for from */, to 1 hour and consequently 
it was observed in which tube the first sign of turbidity due to 
CaC,0, appeared. 
It is clear that the formation of supersaturated CaC,O, solutions 
has to be avoided. 
In cases where the solutions held other salts besides (e.g. RINGER 
solution, ultra filtrate) we have never noticed supersaturation. As a 
matter of fact supersaturation occurred in the case of pure solutions 
of CaC,O,. This can be avoided by setting to work in the follow- 
ing way: 
With a capillary pipet the desired quantities of a, say 0.05, N. 
strong oxalate solution is brought into the dry tubes. In a waterbath 
the tubes are evaporated down to dryness. After this the liquid con- 
taining the Ca” is introduced into the tubes. In this way is prevented 
the formation of already supersaturated solutions. 
For the determination of the calcium ion concentration it is 
moreover necessary to use tubes that are well closed with ground 
glass stoppers. This is necessary to keep the water free of carbonic 
acid or to keep a fixed carbonic acid tension constant. 
It is necessary for the judging of the appearance or non-appear- 
ance of the CaC,O, precipitate that the tubes should be cleaned as 
thoroughly as possible; this can be done in the usual way (chromic 
acid, ABEGG’s steaming process etc.). 
The best way for viewing the tubes is in a box with a slit in 
the bottom from which the light falls through the solution. Care 
should be taken that the light does not fall on the eye of the observer. 
The Tyndall phenomenon makes it possible to appreciate the slight- 
est turbidity. Should the solution before the experiment already 
evince a slight opalescence (not due to CaC,O,) as is sometimes the 
case with serum and ultra filtrate, it is advisable to view the solu- 
tions by red light. The wavelength of this light being too great to 
cause refraction the opalescence is not apparent. The temperature 
during the experiment must of course remain constant. It is therefore 
best to work in a waterbath of constant temperature. 
The results obtained by the above method can be controlled in 
