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substance. Also from a physico-chemical point of view it will be 
desirable to start researches on this subject. 
SUMMARY. 
It was set forth that it is of importance: 
1. To possess by the side of the macro- and mierogravimetric 
analysis, another method which enables us to make quantitative 
determinations of very slight quantities of a substance in a simpler 
and more accurate way. 
2. The method detailed here is based on the principle that after 
the reagent has been added, the precipitate formed is centrifagated 
in a calibrated capillary tube until the volume remains constant and 
can be read off. When the voluine of BaSO, corresponding to a 
SO,-sol. of a known concentration has been determined, then it is 
possible to determine by means of this result the SO,-concentration 
of an unknown sulphate-solution. 
3. In order to make a quantitative determination of SO, we 
always add 2°/, ee of HCI1:1 (concentrated NCI diluted with an 
equal volume of water) to 5 ee of the SO, fluid, and to this mixture 
5 ce of a BaCl, 2 aq-solution of 2.44°/,, containing 3 to 5 drops 
of acetone. The precipitate formed is centrifugated until the volume 
remains the same. 
Whether the 5 cc of fluid contains much sulphate or only a little 
and whether these 5 ce of fluid contain Na, K, Ca, Mg, Cl and PO, 
makes no difference whatever, as regards the results: an n-fold 
quantity of SO, gives an n-fold volume of BbaSO, and the presence 
of the above-mentioned admixtures does not affect the volume of 
the precipitate. 
One division = 0.0004 ce. of the BaSO,-solution corresponds to 
0.000294 grammes of SO,. Mistakes greater than 0.000294 grammes 
of SO, are not made if the method described sub 3 is carefully followed. 
Groningen, April 1916. Physiological Laboratory 
of the University. 
Physics. — “Direct optical measurement of the velocity at the axis 
in the apparatus for Fizmav’s experiment’. By Prof. P. Zeeman. 
(Communicated in the meeting of May 27, 1916). 
For. the comparison with theory of the absolute values of the 
shifts of the interference fringes, which I determined for tight of 
different colours in Frizrav’s experiment, the magnitude of the velocity 
