M. H. Sainte-Claire Deville on Affinity and Heat. 369 



racterizing them simply by the effects which are manifested in 

 our daily chemical operations. I hope, therefore, to be logical 

 in the following reasonings. 



When two gases are mixed which spread uniformly into one 

 another, can the phenomenon of what is called their diffusion be 

 compared to the solution of two liquids in one another ? I do 

 not think so. 



Suppose we place together bisulphide of carbon and white 

 phosphorus at a temperature convenient for the melting of the 

 phosphorus, oil and sulphuric ether, chloride of calcium dissolved, 

 acetic acid and water ; we might obtain very decided changes of 

 physical condition, characterized principally by an elevation or 

 depression of temperature, and by a change of density : in a 

 word, the physical properties of these solutions might vary 

 without the chemical properties being sensibly modified. 



Is this the same with gases ? Experiment shows us, first of 

 all, that gases frequently combine without apparent absorption 

 or disengagement of heat (oxide of carbon and chlorine, chlorine 

 and hydrogen in diffused light). But whenever this disengage- 

 ment of heat can be confirmed, the chemical properties have 

 been profoundly changed. Moreover Gay-Lussac's law proves 

 that gases which contract in combining do not change in volume 

 in the same way as liquids which dissolve. The volume of the 

 combination is always a simple fraction, and often a very small 

 one, of the volume of the elements of this combination. Thus 

 contraction, when it exists, not merely characterizes the combi- 

 nation of the gases, but allows the composition to be deduced 

 therefrom. 



Nothing of the same kind can be remarked in the solution of 

 liquids, where the contractions follow no simple law, and do not 

 enable us to foresee any change in the chemical properties of the 

 elements which interpenetrate by solution ; further, the ther- 

 mal phenomena which may be manifested at the moment in 

 which the liquids interpenetrate do not necessarily imply change 

 of their chemical properties. 



Observing that the physical and chemical properties of bodies 

 cannot be separated in a more absolute manner than can the 

 phenomena of combination and of solution, we conclude, first, 

 that the diffusion of gases is essentially different from the solu- 

 tion of liquids, and therefore that whenever, from the mixture 

 of two gases, there results a calorific phenomenon, there is change 

 of condition, and therefore combination. 



The phenomena observed on the contact of liquids and solids 

 which mutually dissolve each other are far more complex, and 

 deserve a special analysis. 



In attacking this question experimentally a large number of 

 Phil. Mag. S. 4. Vol. 32. No. 217. Nov. 1866. 2 B 



