626 
Proceedings of the Royal Society 
3. Experiments to Determine the Lowering of the Maximum 
Density Point of Water by Pressure. By Professor D. H. 
Marshall, Professor C. Michie Smith, and E. T. Omond, 
Esq. 
( Preliminary Notice.) 
These experiments were begun in January last at the request of 
Professor Tait, and have been carried on since then in his laboratory. 
The subject suggested itself to Professor Tait during his experiments 
on the effects of great pressures on thermometers carried out for the 
“ Challenger ” Expedition Commission, and it was indeed by part of 
the apparatus purchased by him for this purpose that the experi- 
ments were made. The method adopted was of an indirect nature, 
and consisted in determining first the change of temperature pro- 
duced by a sudden change of pressure to which the water was 
subjected. Sic William Thomson has shown that if there he a 
sudden change of hydrostatic pressure on a body whose temperature 
is t and pressure p, there will he a change of temperature depending 
upon the change of pressure, the temperature t , the co-efficient of 
expansion at pressure p and temperature t, and the specific heat of 
the body at that pressure and temperature. In the case we are 
going to consider let t he the absolute temperature of the water, p 
its pressure, e its co-efficient of dilatation at pressure p and tempera- 
ture t , and k its specific heat at constant pressure for the same 
pressure and temperature; let now there he a sudden change of 
pressure from p to p - Sp, in consequence of which there is a change 
of temperature from t to t - St, 
then St cc~ Sp . 
k 
This formula can he absolutely true only for small changes of 
pressure. When water is subjected to great changes of pressure, k 
is possibly a little altered, and we shall show from our experiments 
that e is certainly so. 
Since t and k are always + , on the assumption that k will change 
only very little by pressure, the sign of St will depend upon that of 
e. Thus if e be + (as in the case of water above 4° C.), then for a 
sudden decrease of pressure there w'ill be a decrease of temperature ; 
