TRANSACTIONS OF SECTION A. 449 



initial and final states of the system of bodies, whatever he the nature or the 

 order of the transformations. The extension of this principle to the interaction of 

 the molecules and atoms of bodies on one another is of vast importance in relation 

 to our knowledge of the constitution of matter, Rir it enables us to state that each 

 chemical compound has a distinct le\el or potential Avhich may be called its own, 

 and that when a compound gives up one of its elements to another body, the heat 

 evolved in the reaction is the difference between the heat of formation of the first 

 compound, and that of the resulting product. 



We have become accustomed to regard matter as made up of molecules, and 

 those molecules to be made up of atoms separated from one another by distances 

 which are great in comparison with the size of the atom, which we may regard as 

 the smallest piece of matter that we cau have any conception of Each atom has 

 beeu supposed to be surrounded by an envelope of ether which accompanies it in 

 all its movements. The density of the ether increases rapidly as an atom is 

 approached, and it would seem that there must be some force of attraction betvreen 

 the atom and its ether envelope. All the atoms have motions of translation 

 in all possible directions, and according to the theories of Maxwell and Boltzmann, 

 and the experiments of Kundt, Warburg, and others on the specific heat of 

 vapours, in one-atum molecules iu the gaseous state there is no motion of rotation. 

 According to the theory of Pictet, the liquid state being the first condensation 

 from the gaseous state must consist of at least two gaseous atoms combined. 

 These two atoms are bound to one another through their ether envelopes. Then 

 the solid state results from the condensation of a liquid, and so a solid molecule 

 must consist of at least two liquid molecules, i.e. at least four gaseous molecules,. 

 each surrounded by an atmosphere of ether. M. Pictet imagines these atoms to be 

 centres of attraction ; hence in the solid with four such centres the least displace- 

 ment brings into action couples tending to prevent the molecule from tw^istino; as 

 soon as external forces act upon it. All the molecules constituting a solid wiil be 

 rigidly set with regard to one another, for the least displacement sets in action a 

 couple or an opposing force in the molecules on one another. 



Let us now follow the sketch which M. Pictet has given of changes which we 

 may consider it to undergo when we expend energy upon it. Suppose a solid body 

 is a't absolute zero of temperature, which may be regarded as the state in which the 

 molecules of a body are in stable equilibrium and at rest, the application of heat 

 gives a -sdbratory motion to the molecules of the solid, which increases with the 

 temperature, the mean amplitude of vibration being a measure of the temperature. 

 We may regard the sum of all the molecular forces as the specific heat of the body, 

 and the product of the sum of all the molecular forces by the mean amplitude of 

 the oscillations ; i.e. the product of the specific heat and the temperature will be 

 the quantity of heat or the energy of motion of the body. As more and more heat 

 is applied, the amplitude of vibration of the molecules increases until it is too great 

 for the molecular forces, or forces of cohesion, and the melting point of the solid is 

 reached. Besides their vibratory motion, the molecules are now capable of motions 

 of translation from place to place among one another. To reduce the solidto the 

 liquid state, i.e. to make the amplitude of vibration of the molecules sufficient to 

 prevent them from coming within the sphere of the forces of cohesion, requires a 

 quantity of heat which does not appear as temperature or molecular motion, and 

 hence it is termed the latent heat of fusion. The temperature remains constant until 

 the melting is complete, the heat being spent in bursting the bonds of the solid. 

 Then a further application of heat increases the amplitude of vibration, or raises the 

 temperature of the liquid at a rate depending on its specific heat until the succession 

 of blows of the molecules overcomes the external pressure and the boiling point is, 

 reached. An additional quantity of heat is applied which is spent in changing tlie^ 

 body to a- gas, i.e. to a state of higher potential, in which the motion of translation 

 of the molecules is enormously increased. When this state is attained, the tempe- 

 rature of the gas again begins to increase, as heat is applied, until we arrive at a 

 certain point, when dissociation begins, and the molecules of the separate substances 

 of which the body is composed have so large an amplitude of vibration that_ the 

 bond which unites them can no longer bring them again into their former positions. 



1880. G 



