160 ATTRACTION, 



the application of easy tests, the predominance of the one or of the 

 other, or their equality. On the other hand, they resemble chemical 

 combinations, because the principles cannot be separated by any me- 

 chanical means ; neither repose, agitation or filtration, has any effect; 

 and decomposition, when one ingredient is sensibly more volatile than 

 the other, is effected by evaporation or distillation, or in other cases, 

 by the intervention of an affinity ; or by cold. 



(II.) This class of compounds appears to be intermediate between a 

 mechanical and chemical condition. We seem to need a division of 

 this kind ; it would free us from embarrassment, with respect to the 

 universality of definite proportions, and it is more reasonable to admit 

 such a division than to suppose the existence of innumerable mix- 

 tures of different combinations, in definite proportions, of such things 

 as sugar and water, alcohol and water, &c. 



As no single word expresses their peculiarities, and for want of a 

 better designation, they may be called chemico-mechanical, or me- 

 chanico-chemical compounds. 



There is great variety among chemical compounds, in the degree 

 in which their properties are changed and new properties produced. 

 Thus, it is observed, that although there is in general no resemblance 

 between water and its constituent principles, oxygen and hydrogen, it 

 retains the high refractive power which is characteristic of hydrogen ; 

 and again the ammoniacal salts formed between ammonia and the 

 acid gases, retain a great volatility, although in other respects widely 

 different from their principles ; the muriate and the carbonate of am- 

 monia are striking examples. There is however no difficulty in as- 

 signing such compounds to the class that is strictly chemical, and 

 they would certainly not belong to that which is chemico-mechanical. 

 This last division is very distinctly separated from mere mechanical 

 mixtures ; silicious sand and lead shot, marble powder and powder 

 of clay, among solids ; and oil and water, and water and mercury, 

 among fluids, would never be confounded with the class of chemi- 

 co-mechanical compounds, which we would separate from those that 

 are truly chemical. Nor is there any difficulty with respect to cases 

 of mere superficial adhesion, as between tallow and iron filings, at- 

 mospheric dust and oils, pollen and varnishes and paints, &c. The 

 union is mechanical, and is to be referred clearly to cohesion or ag- 

 gregation. 



Admitting the distinction that has now been attempted to be estab- 

 lished, there can be no hesitation in adopting the doctrine of 



DEFINITE PROPORTIONS. 



(MM.) IN ALL ENERGETIC COMBINATIONS, THE PROPORTIONS OF 

 THE CONSTITUENT PRINCIPLES, WHETHER THEY ARE SIMPLE OR 

 COMPOUND, ARE DEFINITE. 



