sion of the substance to be tested by means of the pipettes, and are in many ways most convenient. They should be tolerably wide at the mouth, and have a well-ground glass stopper, and the graduation line should fall just below the middle of the neck, so as to allow room for shaking up the fluid.

Colorimetric Analysis is also used in assaying. It is based upon the fact that a coloured solution appears the more intense the more of the colouring substance it contains.

If, therefore, a solution containing a certain amount of a substance and being in consequence of a certain intensity of colour, is prepared, it will be possible to obtain the solution under assay of an equal intensity of colour by appropriate dilution.

By measuring the volume of the assay solution and taking into consideration the amount of the standard solution, the quantity of the substance contained in the assay solution may readily be calculated.

CHAPTER IX.

THE ASSAY OF IRON.

THE ores of iron, properly so called, always contain the metal in the oxidised state, and in various degrees of purity.

These ores are the following:

1. MAGNETIC IRON ORE (Fe,O=Fe,O,+FeO). When pure, it contains 72 per cent. Fe.

2. RED HEMATITE, anhydrous sesquioxide of iron (FeO3). When pure it contains 70 per. cent. Fe. It occurs in different varieties.

3. BROWN HEMATITE, brown iron ore, limonite, hydrated sesquioxide of iron (2Fe,O,,3HO). When pure it contains 59-89% Fe, and 14.44 of water. It also occurs in many varieties, Bog iron ore being one of them.

4. SPATHIC CARBONATE, sparry iron ore, crystallised carbonate of protoxide of iron (FeO,CO2). When pure it contains 48-275 Fe. It generally contains some percentages, (2 to 15) of carbonate of protoxide of manganese, and carbonate of magnesia, frequently also some carbonate of

zinc.

5. ARGILLACEOUS IRON ORES, clay or clay-band ironstone, impure earthy carbonate of protoxide of iron. This mineral sometimes resembles compact limestone, sometimes greyish hardened clay. Its great specific gravity, its effervescing on the addition of an acid, and acquiring a brown-red colour on roasting, are sufficient means of identifying it.

The following is the result of an analysis of this class of ore by the Author; the specimen was from Ireland, county Leitrim :

Blackband is a combustible schistose variety of this ore. The following analysis is also by the Author:

6. RED SILICEOUS IRON (silica, water, peroxide and protoxide of iron). This mineral occurs only in rare cases in sufficiently large quantities to be used for iron smelting.

Besides these iron ores the following substances, containing iron and used as fluxes, require assaying: Granite, Chlorite, Basalt, Pyrozene, Amphibole, and also some kind of slags (finery cinder, tap cinder, etc.).

A. THE ASSAY OF IRON IN THE DRY WAY.

It has been already mentioned that iron ores very seldom occur in a pure state, and the ores may be arranged for their assay in the dry way (and also for smelting) into five classes.

1. Iron ores containing silica, lime, and another base, which ores are fusible, per se.

2. Iron ores containing predominantly silica.

5. Iron ores containing a large amount of magnesia; these ores are most difficultly rendered fluid.

The flux used for assaying (and also melting) varies according to the nature of the predominant compound, and the quantity used according to the amount of that compound.

If the composition of the ore is known, it is easy to ascertain the amount of flux necessary to form a slag with the bases or silica present; in most cases an extra quantity of the flux should be added, in order to produce a sufficient volume of slag to cover the button.

According to Dr. Percy,* blast-furnace cinder of the following formula may be taken as a type of the kind of slag desirable:

Its approximate composition per cent. is as under :

The following mixtures of various fluxes, when fused, produce a slag which may be regarded as approximating to

the above composition :

Shale or fire clay 3 Alumina

Lime... 2}

According to Bodemann a compound of 56% silica, 30%

lime, and 14% alumina forms a slag most easily rendered

Percy's Metallurgy, p. 240.

+30% say of alkalies, lime, etc., on account of their fusibility, are taken as equivalent to so much alumina.

fluid, but as it is found that this slag itself is not sufficiently fusible in a small assaying furnace (air furnace), an addition of fluor-spar is given to the mixture, and in some cases (the iron ore being very difficultly rendered fluid), some borax is added, or a mixture of borax and fluor-spar.

An exact knowledge of the mineralogical properties of the iron ores, and a due experience, will enable the assayer to properly adjust the fluxes without resorting to an analysis to find what amount of silica and bases are present.

In some iron works of Germany the following proportions of fluxes are used :

Air-furnaces are best adapted for assaying iron ores where many assays are required. And naked crucibles, either of clay or blacklead, or crucibles lined with charcoal, are employed; the latter are preferable.

The button of metal does not adhere to naked pots, but the slag adheres very strongly; so much so, that it cannot be detached with any degree of accuracy for weighing (which in some of M. Berthier's processes is of importance). Black-lead pots allow neither the slag nor button to adhere, but the former dissolves much argillaceous matter from the pot, so that its weight is greatly increased, and the assay cannot be verified. In naked crucibles, charcoal must always be added to the assay, to reduce the oxide of iron; in which case, if an excess be added, it prevents the button from completely forming, so that globules remain in the slag (with care this may, however, be avoided). Neither do naked crucibles resist the fire as well as those lined with charcoal, because the lining supports the sides when they soften. The charcoal lining also allows the assay to be finished without adding any re-agent to the ore; the button can be readily taken out, because it does not adhere to the charcoal; and lastly, the earthy matters in the ore, which have formed a slag, may be collected and