477 
1910-11.] Isopiestic Expansibility of Water. 
that there is no sudden change in the isopiestics on crossing the critothermal. 
It is a common assumption, however, to consider a substance under pressures 
above the critical as gaseous or liquid according as it is above or below 
the critical temperature. This assumption is made in the following 
general statement. 
The volume and coefficients of expansion of water under pressures above 
the critical pressure have been determined for temperatures reaching in 
some cases to 1000° C. The results show : — 
1. The rate of change of volume with temperature increases continuously 
with the temperature to a maximum the value of which decreases with 
increase of pressure. 
2. While in the liquid state water under high pressures and at tempera- 
tures above 240° obeys the ordinary laws of dilatation of liquids. 
3. Almost immediately after entering into the gaseous state, water at 
high pressures behaves as a substance the coefficient of expansion of which 
is independent of the temperature and decreases with increase of pressure. 
It behaves, therefore, like such gases as oxygen, nitrogen, and hydrogen at 
the same pressures but at lower temperatures. 
I think these results justify the statement that wonderfully consistent 
results can be obtained by using a compression cylinder of the type 
described, and I feel deeply indebted to Professor Des Coudres for the 
facilities and guidance given me in carrying out the experiments. 
{Issued separately May 18 , 1911 .) 
