450 REPOET— 1880. 



Tlie potential of the substances is again raised by a quantity which is proportional 

 to its chemical affinity. Again, we may increase the amplitude of vibration, i.e. the 

 temperature of the molecules, and imagine the possibility of getting higher and 

 higher degrees of dissociation. 



If temperature means the amplitude of vibration of the molecules, then we 

 might expect that only those bodies which have their temperatures increased liy 

 the same amount when equal amounts of heat are applied to them can possibly 

 combine with one another; and so the fact that the increase of temperature bears 

 a fixed ratio to the increase of heat may be the cause in virtue of which bodies 

 can combine with one another. Were other bodies to begin to combine together 

 at any definite temperature, they would immediately be torn to pieces again when 

 the temperature is even slightly raised, because the amplitudes of vibration of their 

 molecules no longer remain the same. This idea of temperature is supported by 

 the fact that a combining molecule of each substance requires the same amount of 

 heat to raise its temperature by the same number of degrees, the atomic weights 

 being proportional to the masses of the combining molecules. The celebrated dis- 

 covery of Faraday, that in a voltameter the work done by an electric cuiTent 

 always decomposes equivalent quantities of diflerent substances, combined with 

 the fact that in the whole range of the physical forces work done is equivalent to 

 the application of heat, is quite in accordance with the view that no molecule can 

 con}bine with another which has not its amplitude of vibration altered by the same 

 amount when equal quantities of heat are applied to both. As soon as we get 

 any divergence from this state of equal motions for equal increments of heat, then 

 we should expect that a further dissociation of molecules would take place, and 

 that only those which are capable of moving together can remain still associated. 



Just as in the change of state of a body from the solid to the liquid, or from 

 the liquid to the gas, a great amount of heat is spent in increasmg the motion of 

 translation of the molecules without altering the temperature, so a great amoimt 

 of heat is spent in producing dissociation without increasing the temperature of the 

 dissociated substances, since the principle of conservation of energy has been shown 

 by M. Bert helot to hold for the dissociation of bodies. We may conveniently make 

 use of the term latent heat of dissociation for the heat required to dissociate a 

 unit of mass of a substance. 



We may thus sum up the laws of physical and chemical changes : — 



1. All the physical phenomena of change of state consist in the subdivision of 

 the body into molecules or particles identical with one another. 



2. The reconstitution of a body into a liquid or a solid being independent of the 

 relative position of the molecules, only depends on the pressure and temperature. 



3. Dissociation separates bodies into their elements, which are of diflerent kinds, 

 and the temperature remains constant during dissociation. 



4. The reunion of dissociated bodies depends on the relative position of the 

 elements, and so depends on the grouping of the molecules. The atomic weight 

 being the mass of a molecule as compared with hydrogen, the specific volume, i.e. 

 the atomic weight di'V'ided by the density, is the volume or mean free ^x/<A of a 

 molecule. 



Building up his theoi-y of heat on these principles, M. Pictet arrives at a de- 

 finite relation between the atomic weight of a body, its density, its meltmg point, 

 and its coefllcient of expansion, which may be stated thus — 



Tlie -Nolurae of a solid body will be increased a? the temperature I'ises by an 

 amount which is proportional to the number of molecules in it, and in\-ersely as its 

 specific heat. At a certain temperature peculiar to each body, the amplitude of 

 the heat oscillation is suflicient to melt the solid, and we are led to admit that for 

 -all bodies the iutermolecular distance corresponding to fusion ought to be the same. 

 Tlie higher the point of fusion of a body, the shorter, on this theory, must be its 

 heat--\-ibrations. The product of the length of swiiu/ (the heat-oscillations) by the 

 temperature of fusion ought to ]x a constant number for all solid bodies. 



A comparison of the -^-alues of the various quantities invoh-ed in these state- 

 jnents shows a very satisfactory agreement between theory and experiment, from 

 T-hich it appears th.at the product of the length of swing by the temperature of 



