and on Dissociation, 453 



stance of elementary or constituent molecules by producing a 

 dilatation, which is very small, and perhaps insensible in refer- 

 ence to the dilatation of the integrant molecules ; but as this 

 distance increases until it becomes equal, then superior to the 

 radius of the sphere of activity of affinity, the compound body 

 decomposes by heat. Between the temperature at which this 

 decomposition takes place, and the temperature at which the 

 body possesses all its stability, there is an interval during which 

 the constituent molecules are at a distance comparable to the 

 radius of the sphere of activity of the affinity. Just as the loss 

 of cohesion suddenly reduces the entire body to a liquid state 

 (if it is solid), so the loss of the stability of bodies under the in- 

 fluence of heat, during this interval of temperature, reduces the 

 molecules to the particular state of superheated bodies to which 

 I have given the name dissociation. The least cause can then 

 produce decomposition ; a mechanical cause, such as platinum 

 decomposing water at 2000°; a simple solvent action, as the 

 solubility of oxygen in silver or in lead, produces the decompo- 

 sition of water at 1000°; lastly, a chemical action, usually elective, 

 but devoid of this character at a high temperature, as iron assi- 

 milating oxygen in the presence of hydrogen and potassium in 

 the decomposition of potass by Gay-Lussac and Thenard's 

 method. 



I may thus affirm that, in this interval of 1000° to 2500°, the 

 constituent molecules of aqueous vapour are free in spite of 

 affinity, just as from 0°to 100° (Cent.) the integrant molecules 

 of liquid water are free in spite of the cohesive force, which is 

 sensibly nil. 



Molecular attraction produces in bodies cohesion, which for 

 each special substance assumes different intensities, the values 

 of which vary with the temperature. There are sudden changes, 

 one of which, corresponding to the liquefaction of solid bodies, 

 takes place at the same time that a considerable quantity of heat is 

 absorbed, without any change in the volume of bodies, while even 

 in some cases there is a contraction, as is the case with water, 

 bismuth, &c. In like manner affinity communicates stability to 

 compound bodies, which diminishes as the temperature increases. 

 This variation, which is insensible within extended limits, may 

 be considerable at a certain temperature, and fixed like the 

 fusing-point, and without the volume of the compound becoming 

 sensibly increased. The compound body (which I shall consider 

 for the moment as being in a gaseous state) is then in a state 

 of dissociation. It could then even absorb a considerable 

 quantity of latent heat of dissociation, without its volume, and 

 consequently the uniformity of its expansion, being notably 

 modified. 



