but is the rapidity with which the charge can be treated.  In the older styles of treatment the time varied from thirty six to ninety hours.  Now this is accomplished in from three to six hours with a practically perfect result.  The older processes required a careful damping of the ore, which, to get good results, must leave the ore neither too wet nor too dry.  Now “damping” is entirely done away with, and in its place water is poured into the barrel.  Pressure to the extent of four atmospheres causes chlorine gas to leave its vaporous form.  Thus the pressure applied not only enables a strong solution of chlorine to be formed with the water in the barrel, but forces this into contact with the gold through every crevice in the ore.  Chlorine gas also takes up any silver which may exist in association with the gold.  In the older processes this is deposited as a film of chloride of silver around the fine gold grains, and from its insolubility in water prevents the absorption of the gold.  The rotary motion of the barrel in the Newbery-Vautin method counteracts this by continually rubbing the particles together; this frees the particles from any accumulations, so that they always present fresh surfaces for the action of the solvent.  Again, the short time the ore is in contact with the chlorine does not allow of the formation of hydrochloric acid, which has a tendency to precipitate the gold from its soluble form in the water before being withdrawn from the chlorinator.

Hitherto, when the ore was very fine or contained slimes, the difficulty of filtration was increased, sometimes in extreme cases to such an extent that chlorination became impracticable.  By the introduction of the vacuum pump this is greatly facilitated; then by making the action intermittent a jigging motion is given to the material in the filter which prevents any clogging except in cases of extreme fineness.

The advantage of using charcoal as a decomposing agent for chloride of gold was pointed out by Mr. Newbery some twenty years ago; four or five years since the idea was patented in the United States, but as this was given gratis to the world years before, the patent did not hold good.  The form of precipitation generally adopted was to add sulphate of iron to the liquid drawn from the filter.  This not only threw down the gold it contained, but also the lime and magnesia.  Then very great care was necessary, and a tedious process had to be gone through to divide the gold from these.  Now, by filtration through charcoal everything that is soluble in hydrochloric acid passes away with the water; for instance, lime and magnesia, which before gave such great trouble.  In passing through the charcoal, the chloride of gold is decomposed and all fine gold particles are taken up by the charcoal, so that it is coated by what appears to be a purple film.

Should copper be associated with the gold, the water, after running through the charcoal filter, is passed over scrap iron, upon which the copper is precipitated by a natural chemical action.  If silver is contained in the ore, it is found among the tailings in the filter, in a chloride which is insoluble in water.  Should the quantity prove sufficiently large, it may be leached out in the usual way by hyposulphites.