1891-92.] On the Thermal Effect of Pressure on Water. 133 
Not© on the Thermal Effect of Pressure on Water. 
By Professor Tait. 
(Read July 18, 1892.) 
I have just seen in the Qomptes Rendus (June 27) an account of 
some experiments, on this subject, made by M. Galopin in the 
laboratory of Professor Pictet. As the effects obtained by him seem 
to be somewhat greater than my own experiments had led me to 
expect, I was induced to repeat my calculations with the view of 
trying to account for the difference. Unfortunately, M. Galopin’s 
work is confined to 500 atmospheres, a pressure which lies a little 
beyond the range of my experiments ; so that no very trustworthy 
comparison can be made. M. Galopin’s results have one advantage 
over those of the direct experiments of the same kind which I made, 
inasmuch as he wms able to use ordinary thermometers, wPile I 
employed thermo-electric junctions, in measuring the rise of tem- 
perature by compression. But they have a corresponding disad- 
vantage, in the fact that mine were obtained instantaneously (by 
means of a dead-beat galvanometer) and required no correction ; 
while his had to be corrected for the heat-equivalent of his apparatus 
to an amount not easy to estimate with accuracy. 
I had assured myself of the general accuracy of my own work by 
showing that three altogether independent modes of estimating the 
effect of pressure on the maximum density point of water gave 
closely concordant results : — viz. a lowering of that point by about 
1° C. for every 50 atmospheres. These investigations were described 
to the Society in 1881-4, and appear (in abstract) in vols. xi. and xii. 
of our Proceedings ; more fully in the Challenger Reports. One mode 
of determination was direct (a modification of Hope’s experiment) ; 
the others were theoretical deductions, from the compressibility of 
water at different temperatures, and from the rise of temjDerature 
produced by compression, respectively. M. Amagat subsequently 
obtained a result very closely agreeing with mine as given above. 
