CHAPTER III.

THE FLUXES, THEIR COMPOSITION, MODE OF PREPARATION AND USE.

In some of the operations in the dry way, bodies are heated in suitable vessels per se; but more often it is necessary to add to the bodies submitted to assay,. other substances which are varied according to the nature of the change to be effected. As these substances generally determine the fusion of the body to which they are added, they are termed fluxes or fusing agents; but this name cannot be applied to all of them with exactitude.

They are generally divided into five classes :— 1. reducing agents; 2. oxidating agents; 3. desulphurating agents; 4. sulphurating agents; and lastly, fluxes properly so called.

REDUCING AGENTS.

All the substances belonging to this class have the power of removing oxygen from those bodies with which it may be combined. They are as follows:

1. Hydrogen gas.

2. Charcoal.

3. The fat oils, tallow, and resins.

4. Sugar, starch, and gum.

5. Tartaric acid.

6. Oxalic acid.

7. Metallic iron, and lead.

Hydrogen Gas.-This body is so termed, because in combining with oxygen it forms water. The most common method of preparing this gas consists in dissolving zinc in dilute sulphuric acid. It is invisible and colourless when absolutely pure, and is the lightest body in nature. It is a most powerful reducing agent, and reduces a great number of metallic oxides at a red or white heat, viz. the oxides of lead, bismuth, copper, antimony, zinc, iron, cobalt, nickel, tungsten, molybdenum, and uranium. When any

metal is required in a state of absolute purity, this is the only reducing agent admissible, as all others give the metal combined with a certain proportion of charcoal.

Carbon.-Perfectly pure carbon is exceedingly rare in nature. It is found in large quantities in the mineral kingdom combined with other bodies. In a perfect state of purity it constitutes the diamond. The diamond, like all other species of carbon, is unacted on by the highest possible temperature when in close vessels. It burns in atmospheric air and oxygen gas, but requires a higher temperature than ordinary charcoal. After the diamond the most remarkable and purest species of carbon are:

Firstly. Black-lead or Graphite, this is a mineral found in beds in the primitive formations, principally in granite and mica-schist. The purest at present known is found at Borrowdale in Cumberland.

Secondly. Anthracite is another species of fossil carbon much resembling ordinary coal, but differing from it by burning with neither smell, smoke, or flame.

Thirdly. Coke is the residue of the coal employed in the gas works after all the volatile matter is expelled. It is generally iron black, and has nearly a metallic lustre it is difficult to inflame, and burns but in small pieces, but gives a very intense heat.

Fourthly. Wood Charcoal is obtained by burning the woody part of plants with a limited supply of air, so as to drive off all their volatile matters, and leave merely their carbon. It is this kind that is generally employed in assays. It ought to be chosen with care, well pulverized, passed through a sieve and preserved in well stopped vessels. Wood charcoal is never perfectly pure, it generally contains a proportion of hydrogen and watery vapour; these bodies are not exactly prejudicial, but in some experiments they ought not to be present; in that case pure charcoal may be readily procured by heating sugar to redness in a close crucible.

Charcoal by itself possesses two inconveniences : firstly, it has the property of combining with many metals; and in the second place it is infusible, and cannot combine with vitreous substances. The property it possesses of combining with iron, nickel, cobalt, &c., is of no consequence to the assayer, for the increase of weight it gives is not material; but its infusibility and inability to combine with fluxes is a very serious inconvenience; for after the reduction, that portion which has not been consumed, remains disseminated with the grains of metal in the fused slag, and

prevents the separation of all the metal, and the consequent formation of a good button; a large quantity of charcoal can thus irreparably injure an assay. This inconvenience does not happen, however, when an oxide is reduced by cementation in a lined crucible; but there are some cases in which its employment is inadmissible.

In default of charcoal, coke may be employed, but it must be chosen with much care, for it often contains a very large proportion of earthy and other extraneous matters; so that before use, it is necessary to burn a quantity, in order to ascertain their amount and composition, and that coke ought only to be used which gives nearly white ashes, and which contains no more than a few per cents. Coke is never so good as wood charcoal as a reducing agent, because it burns more slowly by combining less rapidly with oxygen. When it is used the temperature employed for an assay must be much increased.

Coal is nearly always inconvenient, because it swells by heat; nevertheless as it is not required in very large quantity it is sometimes employed, being previous to use finely powdered and sifted.

THE FAT OILS, TALLOW AND RESIN.

The Fat Oils.-The name oil is generally given to those bodies that are fat and unctuous to the touch, more or less fluid, insoluble in water, combustible and forming soaps with alkalies. They all congeal and become solid at various degrees of temperature. There are even some which, in the temperature of our climate,

have constantly a solid form, as butter, palm oil, cocoa nut oil, &c.

Fixed oils have a very marked unctuosity, they neither dissolve in water nor alcohol, and take fire at a heat capable of reducing them to a state of vapour. Their density is from .915 to .940, and the boiling temperature about 600°. When distilled, they are decomposed, disengaging margaric and oleic acids, &c., then an empyreumatic oil, and lastly a yellowish red substance, and leaving about 2 per cent. of charcoal. When heated to a higher temperature than their boiling point, they are very nearly wholly resolved into an inflamma

ble gas.

According to MM. Gay Lussac and Thénard, these oils consist of:

Tallow is an animal product analogous to the fat oils both in chemical composition and properties; it is soft but solid, white, translucid and granular. According to Chevreuil, mutton fat is composed of:

The Resins.-The greater part of the resins are solid; but some are soft. They are brittle with a vitreous and shining fracture, and often transparent. They are very