578 REPORT— 1886. 



In this paper sucli explanations as have been offered from time to time are- 

 criticised and rejected as inapplicable to ail the phenomena, and an attempt is made 

 to reason out a general explanation on purely thermo-dynamical principles. The 

 initial-temperature of dissociation is referred to the inequality of the atom-tempera- 

 tures (absolute) of the molecules, there being (from a variety of causes) a number 

 of molecules in every mass of gas in a state of greater thermal activity (both as 

 regards translational and internal vibrational energy) than that which corresponds 

 to the mean condition which is indicated by the thermometer. For such molecules 

 a smaller addition of heat is required to bring about dissociation, while the enor- 

 mous increase in the extent of solid surface presented by the introduction of a porous 

 or finely divided solid, increases proportionately the number of instances of impact 

 in a given period of time, and a corresponding increase iu the amount of intra- 

 molecular work done iu promoting the vibrational energy of the atoms. 



The term ' quasi-nascent state ' is used to indicate sucli a degree of tension, thus 

 brought about within some of the molecules, that atoms of opposite electro-chemical 

 energies can, under the influence of their mutual attraction, escape from the mole- 

 cules in which they were previously held, and enter into new combinations. 



Collateral evidence of the truth of the theoretical view here propounded is 

 brought forward, (1) from a purely dynamical example given by Sir W. Thomson 

 in a discussion in Section A last year ; (2) from tl\e action of Mr. Crookes' radio- 

 meter ; (3) from a lecture on dissociation given by Mr. Frederick Siemens in May 

 last at the Royal Institution. 



The general application of the theory is pointed out as claiming more considera- 

 tion than the more empirical explanations hitherto oftered, each of which only 

 applies to a few instances of the kind. It also enables us to refer the contact influ- 

 ence of chemically inert solid bodies, and the action of such purely physical 

 agencies as heat, electricity, and light, in promoting combination, to the same 

 general principles, while it seems to extend our idea of the action of atoms in th& 

 nascent state. 



FlilDAY, SEPTEMBER 3. 



The following Reports and Papers were read : — 



Second Report of the Committee on Vapour Pressures and Refractive 



Indices of Salt Solutions. — See Reports, p. 204. 

 Second Report of the Committee on certain Physical Constants of 

 Solution, especially the Expansion of Saline Solutions. — See Reports, 

 p. 207. 



3. On the Phenomena and Theories of Solution. 

 By Professor W. A. Tilden, F.R.S.See Reports, p. 444. 



4. Water of Crystallisation.^ .B?/ Dr. NicOL. 



The author examines the evidence for and against the existence of water of 

 crystallisation in solution, derivable from thermo-chemical and other data. He 

 shows that there is no relation between heat of hydration of the solid salt and the 

 number of water molecules in the hydrated salt ; that it is not sufficient to deduct 

 the heat of change of state of the water to obtain the true heat of solution of a 

 dehydrated salt ; and that the heat of neutralisation of a dissolved base by a 

 dissolved acid is conclusive against the existence of water of crystallisation in 

 aqueous solution, for with the seven soluble bases this is a constant for HjSO^ and 



' Since published in the Chemical News, p. 191, 1886. 



i 



