begin to rise. Then open muffle sufficient to allow free circulation of air and yet not enough to cool.

If vaporization is slow to begin, pieces of charcoal may be put in front or small bits of pitchy pine a little larger than a match. These will raise the heat enough to start operations. Great care must be used to prevent freezing, i.e., the solidification of the button. If it does freeze raise the heat as just mentioned. You can also have the muffle too hot as well as too cold. A few trials afford the best instructions as to heat required. If after melting the button begins to shoot or splatter up little drops of lead to top of muffle, you will then know that the cupels were not hot enough. As soon as the buttons color, i.e., as soon as the lead is expelled, the fumes will cease and the cooling of the buttons sends rainbow colors over them and the buttons assume

a bright silver color. Remove cupels, mark on the proper number and put new cupels in muffle and repeat operations until assays are finished.

Now comes the final process, the weighing and parting of the buttons. I would recommend to the prospector the substitution of a small ivory scale for the micrometer or the button balance. This scale has holes in the top made to chamber the buttons. It costs $3 or less. Care must be used to see that the button fits the hole. The scale will indicate the value for silver and the value for gold for each sized button of either metal. Note the silver weight then place the button in the parting bottle and put in enough nitric acid to cover the button in bottom of bottle. The larger the button the more acid required.

Set the parting bottle on top of the muffle in a little bone-ash and allow the acid to boil. If enough silver was contained in the button the acid will give off nitrous fumes. Allow it to boil slowly until nitrous or yellow fumes have ceased and the acid is giving off white fumes. Remove bottle, dilute with soft hot water and allow gold to settle, then carefully pour off the liquid, allow the gold to remain in bottom of bottle, in a small compact black mass. Then put your Berlin annealing cup over top of bottle, hold tight and upset bottle and allow the gold to settle

into the cup in a compact mass. Be sure to manipulate cup until black gold powder or sponge is in compact mass on one side of bottom of cup. Carefully drain off the water and set cup on muffle or other warm place to slowly dry. If dried or heated too suddenly there is danger of loss of all or a part of the gold. When heated put cup in muffle and heat gradually till cup is cherry red. Then remove and cool. If you have a button scale the gold being bright is now ready to weigh. If you have the ivory scale or the micrometer you must put the gold in a little cup-shaped piece of lead sheet, close up and run down into a gold button on bone-ash in the blow-pipe cupel holder. Manipulate till button is round, then measure and the assay is complete. In brief, if your ore contains little silver or no silver, you must add silver when you mix your crucible charge. If your button will not dissolve in the acid you can remove it and with the blowpipe add more silver and then part as above. You must have two and a half times as much silver as gold to part the same. The strength of your acid must depend on percentage of gold and silver in button. The more silver you have the weaker your acid should be, for the reason that weak acid leaves the gold in a mass and a strong acid scatters it in the bottle. I always dilute as I put acid in parting bottle, according to presumed need of each individual assay.

Your greatest trials will come in learning the use of the blowpipe. You must inhale breath through your nose and expel through the blowpipe, using mouth as a bellows. The flame must be steady and uninterrupted. You will soon learn how to manipulate the same and to distinguish the difference between the oxidizing and reducing flames and their uses. A common candle makes a fair heat. You must be certain that the lead is expelled from every button, otherwise you will have more silver in results than belongs there and your acid will not part the same. You must subtract the weight of the gold button from that of the button before parting, as otherwise results will show too much silver.

The Wet Copper Assay.-First weigh out 1 grm. of ore, add

quantity of powdered charcoal, increasing to correspond to quantity sulphides in ore, and put in roasting dish if you have one, otherwise on sheet iron. Heat red hot and continuously stir until charcoal is consumed and sulphur all burned out of ore. Cool, repulverize, careful not to lose any of the pulp. Put in glass beaker, cover with nitric acid and heat on piece of wire screen or on sand heated in dish. Allow it to digest, then

Then filter and wash

dilute, filling the beaker, say one-half full. tails through filter a few times so that beaker will be threefourths full. Then add aqua ammonia, stirring the same thoroughly until solution has a strong ammonia odor. If any copper is present the solution will turn blue. Set up your burette stand with burette filled with standardized cyanide solution. Drop into assay solution, thoroughly stirring same until solution turns clear, i.e., loses its blue color. Note the cubic centimeters you have used and reduce by proportion to the copper as shown by the standardization of your cyanide solution.

To Standardize Cyanide Solution in the Field.-Weigh up 1 grm. of copper foil (you can use copper wire if you have no foil), ' dissolve in nitric acid as above. Dissolve cyanide of potassium in quantity1 to make as much solution as you will need at that camp and keep solution in dark bottle in a dark place. Handle with care as this solution is deadly poison. Keep bottle well corked as solution open to air will lose its strength. Put in burette and drop slowly into copper-ammonia solution just prepared and note number of cubic centimeters used to destroy the blue of the solution. If it requires 50 c.c! cyanide solution you will have 50 c.c. solution = 100 per cent. copper-and therefore 1 c.c. solution equals 2 per cent. copper. If it required 10 c.c. cyanide solution to destroy the copper in your assay of the ore you will have as follows: 50 c.c. solution is to 100 per cent. copper as 10 per cent. is to the copper in your assay, which is 20 per cent. copper; or you can multiply your 10 c.c. solution cyanide used by standardized value, which 2 per cent. equals

1 In proportion of 20 grm. to 1 pint of water, for 1 c.c.1 per cent. copper approx.

20 per cent. copper. Thus your ore carries 20 per cent. of

copper.

You can check your copper by fire assay, as outlined in the article above. In the wet assay of some ores, you will find elements that interfere with your assay, but by means of the fire check you can determine the copper contents accurately enough for all your purposes. In fire assays for either lead or copper your charge must, of course, not include litharge or red lead. It may be well for you to experiment on pulp, the contents of which are known until you become familiar with this method. If you wish to pursue assaying in a commercial way you can procure any one or all of a dozen text books which will treat of the things omitted by me. I am trying to help the man who finds a fortune but who may lose it because he may fail to have it assayed through negligence or financial stress.

CHAPTER XIV

GEOLOGICAL PROSPECTING (Continued)

"Well that's an awful lean and hungry lookin' country,' said the Old Prospector.

"Can't tell exactly, but it is, does'nt look as if you would find much there, "shells (shales) an' quartz rock an' bull granite are no 'count anyhow."

In a general way the practical prospector recognizes differences in formations and by long experience comes to note unconsciously the signs which mean a good deal from the geologist's point of view.

Broadly it may be said that hilly or mountainous regions where the formations show folding and fracturing are the most likely places to look for ore deposits. This does not apply entirely to placers, nor to coal measures, and sedimentary deposits generally. The lead and zinc deposits of the Mississippi valley are also an apparent exception. But for the great majority of deposits the rule is good, because (1) there are usually intrusive igneous rocks which we believe to be the primary cause of the mineralization; (2) the intruded rocks have been shattered and fractures formed for the deposition of ores; (3) erosion has been more active and has exposed the outcrops.

Regions in which igneous rocks are in evidence are more favorable, because of the usual close association of ores with these rocks. The acidic, or lighter colored rocks are more often associated with ore bodies than the basic rocks.

The central portions of great intrusive masses are not usually productive and we must look to their margins and especially