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Phosphorus 394 Gravimetric determination 396 Volumetric determination 397 Nitrogen and Nitrates 400   CHAPTER XVIII. SILICON, CARBON, BORON. Silicon and Silicates 405 Carbon and Carbonates 414 Coals 418 Shales 420 Carbonates 424 Boron and Borates 429   APPENDIX A. Table of atomic weights and other constants 433 Table for converting degrees of the centigrade thermometer     into degrees of Fahrenheit's scale 435 Tables showing strengths of aqueous solutions of nitric and hydrochloric acids,     of ammonia and of sulphuric acid 436   APPENDIX B. Estimation of small quantities of gold 440 Practical notes on the iodide process of copper assaying 441 Method of separating cobalt and nickel 442   APPENDIX C. A lecture on the theory of sampling 444     Index 450

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A TEXT-BOOK OF ASSAYING.

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CHAPTER I.

INTRODUCTORY.

Assaying has for its object the determination of the quantities of those constituents of a material which add to or detract from its value in the arts and manufactures. The methods of assaying are mainly those of analytical chemistry, and are limited by various practical considerations to the determination of the constituents of a small parcel, which is frequently only a few grains, and rarely more than a few ounces, in weight. From these determinations calculations are made, which have reference to a mass of material of, perhaps, hundreds of tons. But in all cases, whether the mass under consideration be large or small, whether the material be obtained by mining, grown, or manufactured, the assayer is supposed to receive a small quantity, called "the sample," which is, or ought to be, the exact counterpart of the mass of material that is being dealt with. The taking and making of this sample is termed "sampling"; and the men whose special work it is to select such samples are "the samplers."

But although "sampling" is thus distinct from "assaying," the assayer should be familiar with the principles of sampling, and rigorous in the application of these principles in the selecting, from the sample sent him, that smaller portion upon which he performs his operations.

Sampling.—In the case of gases, there is absolutely no trouble in mixing. The only difficulty is in drawing off a fair sample where, as in flues, the body of the gas is in motion, and varies a little in composition from time to time. In this case, care must be taken to draw off uniformly a sufficient volume of the gas during a prolonged period; any portion of this larger volume may then be taken for the analytical operation.

In the case of liquids, which mix more or less easily—and this class includes metals, &c., in the state of fusion—more or less severe agitation, followed by the immediate withdrawal of a portion, will yield a fairly representative sample.

In the case of solids, the whole mass must be crushed, and, if not already of fairly uniform quality, mixed, before sampling can take place. Most of the material which a sampler is called upon to deal with, is, however, in a more or less divided state and fairly uniform. In practice it is assumed that 5 per cent. of the whole (= 1/20th), if taken in portions of equal weight and at frequent and regular intervals, will represent the mass from which it was taken. Taking a heap of ore, A, and selecting one out of every twenty spade-, bag-, barrow-, or wagon-fuls, according to the quantity of stuff in the heap, there is obtained a second heap, B, containing one-twentieth of the stuff of