EXPEEIJIENTAL KNOWLEDGE OF THE PROPEETIES OF MATTER. 473 



different conditions, in respect of tlie temperatures corresponding to 

 vapour-densities.^ 



In the case of iodine the striking feature is the fact that it can remain 

 for some considerable time at ordinary temperatures in a state in which 

 a large proportion of the two-atom molecules are dissociated. 



In the first part of their report, p. 22, 1886, experiments with solid 

 and liquid benzene by W. Fischer ^ were alluded to, from which Fischer 

 constructed curves for the vapour-pressures at different temperatures in 

 the neighbourhood of the melting-point of solid benzene ; but the curves 

 for solid and liquid benzene did not, as they should, meet at the melting- 

 point of the solid. 



Ramsay and Young have since shown ^ that Fischer's experimental 

 results were sufficiently accurate, bat the construction of his curves was 

 at fault ; for the curves could not be accurately drawn from the formulas 

 he used. On calculating the constants in a Biot's formula 2?= a + &«', the 

 calculated results were more nearly than those found by Fischer's formula 

 in agreement with his experimental results ; and in particular near the 

 melting-point Fischer's recalculated numbers are found to be in close 

 accordance with Ramsay and Young's ; indeed, the agreement is so close 

 as to afford a strong presumption of the purity of the benzene used in 

 both cases. 



Specific Heat and Temperature. 



When a mass m of a body is raised from temperature /j to tempera- 

 ture ^2) under given conditions, the amount of heat consumed in the 



process being Q, the quantity _^ is called the mean capacity of the 



mass for heat between those temperatures ; for example, if t^ be 0° 

 and to = i°, then Q/< is the mean capacity for heat between 0° and t° 

 of the mass m of the given body under the conditions. If, now, for all 

 temperatures from 0° to beyond t, Q should be found to be propor- 

 tional to /, then Q/i* is the capacity for heat of mass m of the body for 

 each degree from 0° to i°, where t is any temperature whatever within 

 the limit; but if, as is in general the case, Q/^ varies as t varies, then this 

 ratio gives no information as to the capacity for heat for the various tem- 

 perature-intervals between f, and t^, but gives for the whole interval ^, 

 to f 2 the mean heat-capacity of mass m. 



The mean capacity for heat for unit mass for interval ct, called the 



mean specific heat, is ^, where ^Q is the heat consumed in raising 



in it 



the temperature of mass in of the body under the given conditions from 

 t to t + ct ; and, as the capacity for heat increases usually with the tem- 

 perature, and the expenditure of heat on a body will in general raise its 



temperature, we have generally a limit value - -^ for the capacity for 

 heat between t and t + H when It has become indefinitely small. 



This quantity — - is the specific heat of the body at t° under the 

 'III dt 



given conditions ; if ct is 1°, — — is undistinguishable from — -yf, 

 ° in dt m dt 



' See first part of this report, 1886, pp. 18 and 40. = Wied. Ann. 38, 1886, p. 400. 

 * Physical Society, Dec. 11, 1886; and Pltil. Mag. Jan. 1887. 



