62 THE VOYAGE OF H.M.S. CHALLENGER. 



Term independent of t :— 



10- 7 (481-21-25p + 2'25p a ) 



Approximate formula : — 



0-00179 / t t- \ 



38+^ I 150 + 10,000 ) 



Minimum compressibility point, probably about 56° C. at atmospheric pressure, is 

 lowered by increase of pressure. 



Average compressibility of solutions of NaCl for the first ]) tons of additional 



pressure, at 0° C. : — 



0-001SG 



36 +2' + * 



1 



where s of NaCl is dissolved in 100 of water. 



Note the remarkable resemblance between this and the formula for the average 

 compressibility of fresh water at 0° C. and p + s tons of additional pressure. 



[Various parts of the investigation seem to favour Laplace's view that there is a 

 large molecular pressure in liquids. In the text it has been suggested, in accordance 

 with a formula of the Kinetic Theory of Gases, that in water this may amount to 

 about 36 tons-weight on the square inch. In a similar way it would appear that the 

 molecular pressure in salt solutions is greater than that in water by an amount directly 

 proportional to the quantity of salt added.] 



Six miles of sea, at 10° C. throughout, are reduced in depth 620 feet by com- 

 pression. At 0° C. the amount would be about 663 feet, or a furlong. (This quantity 

 varies nearly as the square of the depth. ) Hence the pressure at a depth of 6 miles is 

 nearly 1000 atmospheres. 



The maximum-density point of water is lowered about 3° C. by 150 atm. of 

 additional pressure. 



From the heat developed by compression of water I obtained a lowering of 3° C. 

 per ton-weight per square inch. 



From the ratio of the volumes of water (under atmospheric pressure) at 0° C. and 

 4° C, given by Despretz, combined with my results as to the compressibility, I found 

 3 0, 17 C: — and by direct experiment (a modified form of that of Hope) 2° - 7 C. The 

 circumstances of this experiment make it certain that the last result is too small. 



Thus, at ordinary temperatures, the expansibility of water is increased by the 

 application of pressure. 



In consequence, the heat developed by sudden compression of water at tem- 

 peratures above 4° C. increases in a higher ratio than the pressure applied ; and water 

 under 4° C. may be heated by the sudden application of sufficient pressure. 



The maximum density coincides with the freezing-point at —2° "4 C, under a 

 pressure of 2" 14 tons. 



