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double walled copper box lined with thick felt, the inner and the outer 
wall being provided with plate-glass windows in order to enable us to 
take readings along the whole scale of the tube (comp. PI. {I fig. 4). 
The space between the two walls was filled with water and the 
constancy of the temperature was promoted by stirring. 
The tube to be calibrated rests on a wooden ring and the lengthening- 
piece with stopcock, welded on to the upper-reservoir, passes through 
an india rubber stopper cut in two. The point of the manometer- 
tube through which the mercury flows is protected from variations 
of temperature by a copper felt-lined cap fastened to the box by a 
bayonet-adjustment; this cap can easily be removed (for a short 
time) whenever we want to let a quantity of mercury flow out from 
the tube. 
The readings for determining the normal volume are made while 
the manometer-tube is placed in a double-walled box as described 
above, (comp. Pl. II fig. 5) but in which the windows were only 
small, as we wanted to read only the position of the mercury in 
the U-tubes and of the thermometer. These readings and that 
of the standard barometer (the box communicating with the atmo- 
sphere by a small tube) yield a perfectly accurate determination of 
the normal volume, which is of the greatest importance for the 
investigation of the isothermal lines. 
Physics. — Prof. vaN DER WAALS presents on behalf of Mr. N. 
Quint GZN. a paper on: „The determination of isothermals 
for mixtures of HCl and C,H,.” 
Introduction. 
At the commencement of this investigation there were but few 
observations made, which might be used for testing Prof. VAN DER 
Waats’s theory on the behaviour of mixtures of two substances. 
At that time Mr. KuENEN was the only one who had examined 
some mixtures and his observations agreed with that theory. In 
order to add to the material on this subject (to which also Mr. vAN 
DER LEE, Mr. VerscrarreLT, Mr. HARTMAN have since contributed), 
I have examined mixtures of HCl and CH. The results of the 
determinations of the isothermals and a short description of the 
experiments follow; I hope soon to publish some further details and 
a calculation of volume-contraction ete, 
