M. H. Sainte-Claire Deville on Affinity and Heat. 367 



unknown cause, at least when it is not merely the expression of 

 a quality of matter ; in this case it should merely serve to desig- 

 nate the fact that such and such substances can or cannot com- 

 bine under certain well-defined conditions. 



One point is very urgent ; that is, to define combination itself. 

 Seeking attentively the state of our knowledge in this respect, 

 I find, in the first place, that the definition of combination must 

 comprehend solution, and can only exclude mixture. In fact 

 the phenomena of solution and of combination are joined by a 

 continuous chain which it is impossible to break at any one point. 

 Every one knows that interminable discussions have been raised on 

 this subject, — for instance, in respect to metallic alloys and their 

 liquation; in reference to salts, acids, energetic bases, and their 

 action upon water, alcohol, &c. The most certain conclusion 

 which can be drawn is, that there are all possible intermediate 

 stages between the phenomena of combination and^the best-cha- 

 racterized phenomena of solution. 



In endeavouring to formulate in the clearest manner the ideas 

 now current on combination, I find it cannot be better defined 

 than by the fact of change of state. When two substances put 

 in presence change condition, they combine. This change of 

 condition, defined in the ordinary manner, shows itself by some 

 new property, whether physical or chemical, which discloses new 

 qualities of the combined substances — qualities, whether phy- 

 sical or chemical, which distinguish the combination from simple 

 mixture. I will give an example. 



The question has been asked, Is air a mixture or a combina- 

 tion ? How has this question been solved ? The physical and 

 chemical properties of the elements of air (oxygen and nitrogen) 

 and of air itself have been successively studied. They have been 

 found to be identically the same, always equal in the case of air 

 to the mean of these properties determined upon each element 

 singly. Hence it has been concluded that nitrogen and oxygen, 

 coming into contact under ordinary atmospheric conditions, exert 

 no appreciable action on one another, that, therefore, the condi- 

 tion of the gas has undergone no change — that, in short, air is a 

 mixture and not a combination of nitrogen and oxygen. 



It must be observed that recourse has always been had suc- 

 cessively to the physical properties of air to solve so important 

 a question. Biot went so far even as to adduce the equality 

 between the refractive indices of air and of the mean of the in- 

 dices of oxygen and of nitrogen as a proof in favour of the opi- 

 nion now generally admitted. If, in fact, there had been found 

 in a constant manner a measurable physical property, different in 

 air and in a mixture of oxygen and nitrogen, the conclusion would 

 have been immediately drawn that air was a combination. There 



