( 482 ) 
causes taken into account) as the only certain physical criterium 
of purity. 
As long as it has not been proved that existing impurities cannot 
account for the phenomena quantitatively, 1 see no reason to aban- 
don the thesis that each substance shows a critical point at which 
the two coexisting phases become identical, so that one single critical 
density belongs to the critical temperature and the critical pressure. 
Geodesy. — “Determinations of latitude and azimuth, made m 
1896—99 by Dr. A. PANNEKOEK and Mr. R. Posrnumus Mrysrs 
at Oirschot, Utrecht, Sambeek, Wolberg, Harikerberg, Sleen, 
Schoorl, Zierikzee, Terschelling (the lighthouse Brandaris), 
Ameland, Leeuwarden, Urk and Groningen.” Short account 
of the report published under this title by Prof. J. A. C, 
OUDEMANS. 
Besides the stations mentioned in the title, the programme, as 
drawn by the Dutch Geodetic Committee, contained also the stations 
Leyden and Ubagsberg, where the observations were made under 
superintendence of Prof. H. G. VAN DE SANDE BAKHUYZEN, who himself 
will publish them. 
The observations of Messrs. PANNEKOEK and Postaumus MryJzs at the 
above named thirteen stations, have been made under my super- 
intendence, and in an introduction I have given an account and a 
criticism of them. Here the following details may suffice : 
The mean latitude of the four northernmost stations, Terschelling, 
Ameland, Leeuwarden and Groningen is 53°18'39", that of Schoorl, 
Urk and Sleen 52°42'45", that of Leyden, Utrecht, Wolberg and Hari- 
kerberg 52°10'40", that of Zierikzee, Oirschot and Sambeek 51°35'51", 
while the latitude of the southernmost station Ubagsberg is 50°50'53". 
The entire are of meridian, of which the length will be computed 
as soon as the results of the entire triangulation will be known, 
amounts therefore to 2°27'46" and may be considered to consist of 
four parts of 35/54", 325", 3449" and 44/58" respectively. Thus 
it will appear afterwards whether the curvature of the meridian, as 
found here, agrees with the form adopted. 
The Universal instruments used for the observations were of 
Rersorp; they were provided with a horizontal circle of 315 mms., 
and a vertical circle of 245 mms. in diameter, and belonged to the obser- 
vatories of Leyden and Utrecht respectively. The circles were gradu- 
ated to 4, whereas the microscopes of the Utrecht instrument are 
read directly to 2", those of the Leyden instrument to single seconds. 
