DIFFERS FROM CHEMICAL AFFINITY. 25 



a detachment of the solid crystalline particles from the mass of the dissolving substance; 

 these pass among the interstices of the liquid, and remain there, unaffected by gravity, 

 being equally and uniformly diffused. Of the powers by which this is brought about we 

 are not well informed, but no fact in science is better ascertained than this uniform and 

 equable diffusion. If, by affinity, we mean a power that causes substances to unite with 

 an interchange of elements, or one which is only exerted to bring about an alteration of 

 composition, such a force is obviously insufficient to give rise to these effects. 



70. That one particle has the power of attaching itself to another of a dissimilar 

 kind, without anything like change of composition, numerous facts demonstrate. The 

 delicate dyes that adhere to cloth-fibre offer an example ; they cannot be supposed to bo 

 attached by any force affecting either their composition or structure, since the successful 

 operations of the artist proceed upon the supposition that the tint shall be unimpaired, 

 and the strength and organization of the fibre which is dyed shall remain untouched. 

 Now in those cases where we know that the dyeing material acts chemically on the 

 fibre, is there not abundant proof that the elementary changes affect the uniting bodies ? 

 Is not the hue of the dye changed, and does not the fabric become rotten 1 Other 

 facts also show that these adhesions, without chemical change, are possible ; the foil 

 on the back of a mirror is not retained by the exercise of any force which has brought 

 about a change in its composition. When the dye is washed off, or the foil scraped 

 away, the cloth-fibre and the looking-glass are both found in their original integrity of 

 structure. 



71. The cases here cited furnish examples of one solid uniting to another in a man- 

 ner that involves something different from the action of chemical affinity. There 

 is a whole range or class of similar combinations : a solid may unite thus with a liquid, 

 as sugar and water ; a liquid with a liquid, as alcohol and water ; a liquid with a gas, as 

 carbonic acid and water ; or a gas with a gas, as oxygen and nitrogen. All these are 

 cases where there is no interchange of chemical elements, and which we cannot, there- 

 fore, suppose to ensue in virtue of chemical force. 



72. Although these actions are the result of a kind of adhesion of particle to particle, 

 and might, therefore, be supposed to take place in an indiscriminate or irregular man- 

 ner, there are some remarkable circumstances attending them which go to show the 

 contrary; thus, water will dissolve a certain quantity of sulphuric ether, and no more; 

 it will take up its own volume of carbonic acid, and no more ; it will hold in solution 

 of bisulphate of potash, sulphate of ammonia, protosulphate of iron, bicarbonate of 

 potash, chromate of potash, muriate of strontian, &c., half its weight, at 60 F. At the 

 same temperature it dissolves its own weight of sulphate of magnesia, and this com- 

 parison might be extended much farther. The same kind of predilection for definite 

 quantities obtains also in gases, as is the case with atmospheric air, where the propor- 

 tions of oxygen dissolved in nitrogen are as one to four, nearly. 



73. All these things go to prove that the passage of the particles of one body among 

 the particles of another proceeds upon certain and definite laws. Whether the resi- 

 dence of saline atoms among the interstices of a liquid is a phenomenon of the same 

 sort as the adherence of dye to a fibre, it is not material to inquire. We know, by 



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