XVIIL] SOLUTION AND THE CHEMICAL PROCESS. 449 



nary solutions as instances of chemical union ; * and Mr. J. J. 

 Griffin has insisted upon the same view.t As these writers 

 have not, however, sufficiently dwelt upon the important prin- 

 ciple, 'rejected by so many names of authority, that all solution 

 is chemical union, we propose to offer some considerations upon 

 aqueous solution, and endeavor to show that the process pre- 

 sents all the phenomena of chemical combination. First, in, 

 the fact that the resulting saturated solutions are perfectly 

 homogeneous. Secondly, in the condensation and more or less 

 perfect identification of volume (ante, page 428) observed in the 

 process (some anhydrous salts dissolve in water without in- 

 creasing its volume). Thirdly, in the change of temperature 

 which attends the process ; thus oil of vitriol, hydrate of 

 potash, and many anhydrous salts evolve heat when dissolved 

 in water, while sal-ammoniac, nitre, and many hydrous salts 

 produce cold by their solution. Fourthly, in the change of 

 color which attends the solution of some salts, as the chlorides 

 of nickel, cobalt, and copper. 



It must not be forgotten that the liquid state of these aque- 

 ous combinations is often an accident of temperature; alum 

 and the rhombic phosphate of soda are liquids at 212 F., and 

 bi-hydrated sulphuric acid is a crystalline solid below 46 F. 

 The ease with which many of these compounds are destroyed 

 by evaporation, and even by changes of temperature, is not to 

 be urged as an objection to the chemical nature of the union. 

 "We need only compare the corresponding silver-salts with the 

 chloride and iodide of gold, or the hydrochlorates of morphia 

 and ammonia with those of caffeine and piperine, which lose 

 their acid by a gentle heat, to learn how variable is the stability 

 of admitted chemical compounds. Chemical affinity may be 

 very feeble in degree. 



According to Gay-Lussac, one part of oil of vitriol will ab- 

 sorb from air saturated with moisture fifteen parts of water, 

 or more than eighty equivalents ; terchloride of arsenic re- 

 quires eighteen equivalents of water to dissolve it, and the 



* Elements of Chemistry, 7th ed., p. 139. 



t L., E. and D. Phil. Mag. (3), Vol. XXIX. p. 299. 



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