134 SCIENTIFIC APPARATUS. 



should be inclosed in solid vessels, themselves alterable in volume 

 by heat, the determination of absolute dilatation of a liquid 

 requires in general a knowledge of the cubical dilatation of the 

 containing vessels. 



The Pressure of the Ideal Perfect Gas at constant volume is 

 proportional to the absolute temperature. There are various modes 

 of measuring the pressure of a gas at constant volume in terms of 

 the temperature-indications of ordinary thermometers. 



Co-efficient of Cubical Dilatation of a Gas at constant pressure. 

 [Rudberg, Magnus, Regnault, &c.] 



Contraction, by heating, of stretched India-rubber, of water 

 between o C and 4 C, of iodide of silver, and of various other 

 anomalous bodies. 



(b) CHANGE OF MOLECULAR STATE. 



The usual agent in all melting, boiling, evaporation, and dis- 

 sociation is certainly Heat though, under peculiar circumstances,, 

 pressure is found to produce very singular analogous effects. 



MELTING OF SOLIDS. 



The pressure remaining the same, there is a definite melting point 

 for every solid; and (provided the mass be stirred), however much 

 heat be slowly applied, the temperature of the whole remains at the 

 melting-point till the last particle is melted. 



This is one of the bases of Black's doctrine of Latent Heat. 

 Our modern knowledge that heat is not matter leads us to regard 

 the energy which disappears to the thermometer as being employed 

 in tearing asunder the particles of the solid. But the first clause 

 of the statement leads to the important question of the influence 

 of pressure upon the melting point. This was first discussed by 

 James Thomson, in 1849, and his calculations with regard to the 

 lowering of the melting point of ice by pressure were exactly 

 verified experimentally by Sir W. Thomson in the same year. 

 Hopkins, Bunsen, and others, have since extended the experi- 



