GOLD IN SCIEISrCE AND IN INDUSTRY. 229 



amount would not be an excessive estimate of the range in the case of 

 gold. If the range for gold be taken as 500 micromillimeters. then 

 the gold molecules of the dilute gold solution, which are spaced at 

 400 micromillimeters jpart. are just within the range of each other's 

 attraction, and their distribution is, therefore, likely to be uniform. 

 But by a further dilution to half concentration, the equilibrium would 

 be liable to l)e disturbed, and denser clouds of gold molecules would 

 be formed, with less dense intervals between them. 



In prej)aring the zinc boxes through which the gold solution is 

 passed, very great care has to be exercised to insure that the contact 

 surface of the zinc is used to the best advantage. With this object 

 the i^acking of the zinc shavings is so managed that the solution is 

 spread over the zinc surface in as thin sheets as possible. The object, 

 of course, is to bring as many of the gold molecules as possible into 

 actual contact with the zinc. The gold molecules found in the solu- 

 tion leaving the boxes are those which have not l)een in contact with 

 the zinc. Yet we have seen that these molecules are still so numerous 

 that the}" are within eo^^TrTr of an inch of each other. If these mole- 

 cules are in a state analogous to the gaseous state, with diffusive 

 energy of the same order as that of the gas molecule, it is difficult to 

 imagine how they can. escape without coming in contact with the zinc 

 surface during their tortuous passage through the boxes and being 

 deposited there. Yet they do escape, even when the A-elocity of the 

 solution in passing over the zinc surfaces is so slow as 10 centimenters 

 per minute, or 1 •(') millimeters per second. 



AVe may regard the condition of these isolated gold molecules, or 

 the more complex auricyanide of potassium molecules, as typical of 

 that of the solute molecules in a dilute solution of any nonvolatile 

 solid. They are i^olul molecules sparsely distributed among a multi- 

 tude of intensely acti^'e solvent molecules, the temperature of the solu- 

 tion being many hundred degrees below that at which they could 

 of themselves assume the greater freedom of the liquid or gaseous 

 state. These solute molecules have to a great extent been set free 

 from the constraining effect of their cohesive forces, hut if is im- 

 portcoit to rementher tliat this freedom has not been attained htj-Jhe 

 increase of their oivn kinetic energy as in liquefaction hij heat. Their 

 freedom and the extra kinetic energy they have acquired have in 

 some way been imparted to them by the more active solvent molecules; 

 for. if the solvent could be suddenly removed, leaving the solute mole- 

 cules still similarly distributed in a vacuous space, they would even- 

 tually condense into a solid aggregate. This must be the case, for 

 the nonvolatile solute has no measurable vapor pressure at the tem- 

 perature of the solution. The kinetic energy of the solute molecules 

 is of itself quite insufficient to endow them with the properties of the 



