88 A TREATISE ON METAMORPHISM. 



produce more or less of the substances A and B. a That is, the reaction is 

 reversible to a greater or less degree. To illustrate, if two solutions, one 

 of them containing MgS0 4 and the other Na 2 C0 3 , come together, the ions 

 are Mg, Na, S0 4 , and C0 3 . A part of the Mg will unite with the C0 3 , 

 producing MgC0 3 , and a part of the Na will unite with the S0 4 , producing 

 Na 2 S0 4 . Vice versa, if solutions of Na 2 S0 4 and MgC0 3 are mingled in a 

 similar manner, MgS0 4 and Na 2 C0 3 will be produced. The reversible 

 reaction may be briefly expressed thus: 



MgSO^NajCOa^MgCOs+NasSOj. 



The sign Zl means that the equations may be read from left to right 

 or from right to left. 



These are the facts: The ions do interchange between compounds 

 whenever a chemical reaction takes place. Just how and why they inter- 

 change is another matter, upon which there is not agreement. Under the 

 theory of dissociation the interchange takes place through the medium of 

 the free ions. The free ions of a compound A are held to collide with the 

 free ions of the compound B, and thus produce the compound C and D, 

 and vice versa. Or, in the specific case above given, of MgS0 4 and Na 2 C0 3 , 

 the free Mg ions collide with the free C0 3 ions and produce MgC0 3 , and 

 the S0 4 ions collide with the Na ions and produce Na 2 S0 4 . From the 

 MgC0 3 and Na 2 S0 4 , MgS0 4 and Na 2 C0 3 are reproduced in a similar manner. 

 Whether the theory of chemical reactions through free ions be of any value or 

 not, it seems probable that free ions exist for a moment when the interchange 

 takes place. To illustrate, it seems hardly probable that the Mg is united 

 to the S0 4 and the C0 3 at the same time. If the Mg lets go of the S0 4 to 

 attach itself to the C0 3 , for that instant the ion Mg is free. The same is 

 true of each of the other ions, Na, S0 4 , and C0 3 , at the instant of inter- 

 change. Hence the question at issue is the cause of the interchange. Does 

 it take place as the result of contact of free ions jDroduced by dissociation, 

 or does the chemical affinity of the Mg for the C0 3 cause a portion of it to 

 leave the stronger acid radical S0 4 for the weaker acid radical C0 3 , etc.? 

 But this is a question for chemists to settle. The problem is stated here 

 because it is one of such fundamental importance in metamorphism. 



But whatever the cause, reversible reactions are a certainty, and it will 



"Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macniillan & Co., London, 1895, 

 pp. 356-357. 



