238 



Prof. H. Hennessy on the 



where /jl is the coefficient of cubic compressibility, H the 

 pressure of the atmosphere, and a the density of mercury. 

 But, as in solids, the modulus of elasticity is inversely as the 

 compressibility k, we have 



a / Wi 

 V koEo 



kpHa 



Both in solids and liquids the velocity of sound is inversely 

 as the square roots of the densities and compressibilities. 

 Although such solids as metals and rocks are denser than 

 most liquids, the limits of their elastic compressibility are so 

 much less, that sound is propagated far more quickly through 

 such solids than through liquids. In steel and metals gene- 

 rally this has been long since established. In rocks the 

 velocity of sound has been computed from direct experiment 

 by Mallet, and has been found to be greater in continuous 

 homogeneous rock than the velocities observed in liquids'*. 



(4) If we had not the results of direct experiment on the 

 compressibilities of liquids and solids to assure us that these 

 properties in liquids are in excess of those obtained for solids, 

 we might fairly infer this conclusion from the relative dilata- 

 bility of such substances under differences of temperaturef . 

 The construction of our common thermometers is based on 

 the greatly superior dilatability of the liquids enclosed in the 

 thermometer- tube over the material of the tube itself. The 

 dynamical theory of heat clearly establishes that the expan- 

 sion of solids and liquids is a mechanical action as much as 

 their compression under the action of force, and the substances 

 which contract least by cooling are precisely those which con- 

 tract least under pressure. Gases which contract most by 

 pressure are also the most dilatable by heat. Liquids occupy 



* See f Philosophical Transactions ' for 1861 and 1862. 



t Expansions of Metals and Glass for 1° Centigrade, according to 

 Dulong and Petit, at different Temperatures T. 



Solids. 



T. 



Liquid. 



Platinum. 



T. 



Iron. 



T. 



Copper. 



T. 



Glass. 



Mercury. 



1 



o 



100 

 311 



1 



o 



100 



372 



1 



19,400 



1 



o 



100 



328 



1 

 38,700 



1 

 36,800 



1 



o 



100 

 213 

 353 



1 



5550 



1 

 5425 



1 

 5300 



37,700 



1 



38,300 



28,200 



1 



22,700 



17,700 



32,000 



