516 EEPOET— 1888. 



have shown, can be obtained at constant temperature by diminishing 

 pressure, a case of which is a point on the isothermal, for 181 '4°, of 

 alcohol.' Other examples are methyl alcohol, liquid SO,, liquid NH3, 

 heated with their vapour to temperatures considerably above the 

 temperature at which the vapour-pressure is, in the static method, in 

 equilibrium with the atmospheric pressure ^ (this Report, 1886, p. 13). 



Points on the upper part of the sinuous curve — concave to the axis of 

 volumes^are points at which the pressure is in excess of the vapour- 

 pressure for a given temperature ; these points are reached in cases where a 

 space is supersaturated with vapour. In 'Nature,' March 1, 1880, Mr. John 

 Aitken gives an abstract of a paper read by him at the Royal Society of 

 Edinburgh on February 6 the same year. In this remarkable paper, ' On 

 the Number of Dust Particles in the Atmosphere,' he shows that when an 

 expansion by a stroke of the piston of an air-pump is made in the air 

 saturated with moisture in the receiver, a fog is formed by the dust 

 particles acting as nuclei to small droplets of the water precipitated on 

 them through slight loss of heat and lowering of temperature consequent 

 on expansion against pressure ; but that if the air be by filtration 

 through cotton- wool freed from the greater part of the dust particles, 

 after allowing the fog produced by the stroke of the piston to subside,, 

 subsequent fogs so produced will at length become slighter, until a small 

 stroke (and then a larger stroke as the operation is repeated) ceases to 

 produce a fog or any deposition of water in the liquid state ; the air is 

 thus to a greater or less extent supersaturated with aqueous vapour. If 

 all the air-dust has settled, some points on the upper part of an 

 isothermal curve of water are thus obtained, at which the pressure is 

 greater than the vapour-pressure of water at that temperature. 



All points on this sinuous curve represent unstable states of 

 equilibrium, so that if the stroke of the piston is made with a jerk a 

 copious shower is produced in the dust- free air. 



Projperties of Bodies for lohicli i^-=-VY — a. 



Professor G. F. Fitzgerald ^ discusses the ' thermodynamic properties* 

 of substances whose intrinsic eq nation is a linear function of the pressure 

 and temperature.' The intrinsic equation is the equation jt>=iT — a, in 

 which a and h are functions of the volume only ; and the conclusions he 

 draws are the following : — 



(1) The specific heat of the substance at constant volume is inde- 

 pendent of the pressure. 



(2) The internal energy of the substance is the sum of two terms, of 

 ■which one is independent of the pressure, and the 'other independent of 

 the temperature. 



(3) The entropy of the substance is the sum of two terms, of which 

 one is independent of the pressure, and the other independent of the 

 temperature. 



IsopyJcnics. 



The above name is given by Wroblewski ■• for lines representing the 

 relation between pressure and temperature -when the density is kept 



' Phil. Trans. 1886, Part I. plate 3. ' 77^^^ j/^;,„, t. xxvl. p. 645. 



=• Proo. Poi/. Soc. 42, 1887, p. 60. 



* Wieno- Monatshrftf. Chemie, 1886, p. 383 ; and Wied. Ann. 29, 1886, p. 428. 



