then verified by an examination of the weights themselves. This is conveniently done whilst replacing them in the box, which should be done immediately after each weighing.

In some cases, where great accuracy is not of so much importance as rapidity in getting out approximate results, a plan may be adopted recommended by Mr. F. F. Mayer in the American Journal of Science and Art' for 1861.

Mr. Ch. Mène, of Creusot, gave (in the Journ. de Pharm. et de Chimie, for October, 1858) a mode of weighing which does away to a great extent with the tediousness and difficulties attending the drying of many precipitates. washes the precipitate thoroughly by decantation, and then introduces it carefully into a bottle the exact weight of which when filled with distilled water at a certain tem

He

perature is known. Since the precipitate is heavier than water, the bottle when filled again will weigh more than without the precipitate, and the difference between the two weights furnishes the means of calculating the weight of the precipitate.

In case the precipitate settles but slowly, it may be collected on a filter, and, together with a filter, after washing, be introduced into the bottle, in which case the weight of the filter and its specific gravity, supposing any difference should exist between its own and that of water, is to be taken in account. Precipitates soluble in or affected by water may be weighed in some other liquid.

Mr. Mayer has applied this principle on a large scale as far back as 1855.

In that year he was engaged in the manufacture of carbonate of lead from refuse sulphate of lead, by treating the latter, in a pulpy condition, with carbonate of soda. The sulphate of lead used contained very varying proportions of water and soluble impurities; from which latter it had first to be freed by washing. It was then in the state of a thin pulp, and the difficulty was to find the amount of the dry sulphate of lead, as it was a matter of importance to use a little. carbonate of soda, and to obtain as pure a carbonate of lead and sulphate of soda as possible. This could only be done by weighing it as whole or in portions; but as the

drying of a tubful of sulphate of lead (from 500 to 1200 lbs.) was impracticable, and sampling not less so, since the upper strata contained a much larger proportion of water than the lead at the bottom, the following method was contrived, which enabled the management of the process to be left in the hands of a workman :

A strong oaken pail was taken, weighing 8 lbs. when empty, and a black mark was burnt in horizontally around the inside of the pail two inches below the rim, up to which mark it held 20 lbs. of water. The specific gravity of sulphate of lead being 6-3, the pail, if filled up to the mark, would hold 126 lbs. of pure sulphate of lead. The specific gravity of water being 5.3 less than that of sulphate of lead, it followed that if there were 1 lb. of water in the pailful of moist sulphate, the pail would weigh 5.3 lbs. less than 126 (+8, the tare of the pail)=120·7(+8); if there were 2 lbs. of water present, the weight would be 115.4(+8), and so on. This enabled a table to be calculated giving in one column the actual weight of the pail when filled with moist sulphate, and opposite, in a second column, the amount of dry sulphate corresponding to the gross weight. The weight of dry sulphate was thus found as accurately as could be desired, although the amounts varied in practice from 30 to 105 lbs.

This is nothing but an application of the Archimedean theorem, that when a solid body is immersed in a liquid, it loses a portion of its weight, equal to the weight of the fluid which it displaces or to the weight of its own bulk of the liquid.

This is precisely the principle applied by Mr. Mène. The precipitate he obtains by a certain chemical manipulation, is a substance of known composition and specific gravity. Supposing it to be sulphate of lead, and the bottle when filled with water at the normal temperature to weigh 70 grammes 50 grammes of water and 20 for tare. After introducing the precipitate and filling again with water it weighs 71-06 grammes. Now as the specific gravity of sulphate of lead is 6:3, or as the weight of a cubic measure of sulphate of lead is 6-3 times that of a cubic measure of

=

water, and as the space of one part by weight of water is taken up by 6-2 parts by weight of sulphate of lead, it follows that the quantity of the sulphate of lead in the bottle, which has taken up the space of one part by weight of water, increases the original weight of the bottle (filled with pure water) by 5.3. To find the amount of water displaced it is only necessary to divide the overweight (1 06 grammes) by 5.3=0·2, which added to the overweight 1.06 +0.2 gives 1.26 grammes as the weight of the precipitate.

Hence the rule, which is of great convenience in volumetric analysis, that to find the weight of a moist precipitate which is a compound of known specific gravity, weigh it in a specific gravity bottle or some other vessel of known weight when filled with water, or any other liquid at the normal temperature; again fill it with the water or other liquid, divide the excess of the new weight by the specific gravity of the substance, less that of the water or other liquid (that of water being=1), and add the quotient to the overweight, which gives the weight of the precipitate.

The principle exemplified by Mr. Mayer may not be novel; but as it has never been fully exemplified before, chemists and assayers, as well as manufacturers, especially of colours, will probably also find it of interest, and certainly highly practicable and easy of execution.

38

CHAPTER III.

GENERAL PREPARATORY CHEMICAL OPERATIONS.

CALCINATION. Strictly speaking the term calcination means the production of an oxide or cala by combustion, and it necessarily involves the intervention of atmospheric oxygen. But in a metallurgical sense the term is restricted to the separation of any volatile matter from a mineral substance by the aid of heat alone, the atmosphere being totally or partially excluded, or the production of rapid changes of temperature, so as, for instance, to render minerals more fragile by quenching in water, &c.

Thus, we speak of the calcination of minerals, as iron, or zinc, ores, &c., whose matrices are argillaceous, to expel water; and also of gypsum to expel water; the carbonates of lime, iron, copper and lead, are calcined to separate carbonic acid; the hydro-carbonates of zinc and iron, to get rid of both water and carbonic acid; cobalt, nickel ores, &c. to separate arsenic and sulphur; the iron ore found in the vicinity of collieries, to expel bituminous matter; and wood and bones to expel volatile organic matter. Where the operation is accompanied by combustion, and requires the oxygen of the atmosphere, it is termed roasting.

Crucibles are conveniently used in calcination, as no stirring of the mass is required. They may be made of various materials, as clay, plumbago, platinum, silver and iron. Silver cannot be employed at a heat greater than dull redness. The selection of the crucible must depend upon the substance under operation; they must all be furnished with covers.

In almost all operations in assaying it is necessary to estimate the amount of volatile matter lost by calcination. A very high temperature is seldom required in calcination

usually an air furnace will give enough heat. When the operation is finished, the crucible must be removed from the fire and allowed to cool gradually. When completely cold, remove the cover and take out the contents by means of a spatula. If any adhere, a small brush will be found very useful for its removal. The difference in weight before and after calcination will represent the volatile matter.

When the substance to be calcined is fusible, the crucible and contents must be weighed before ignition; and the loss of weight is equal to the quantity of volatile matter expelled; in fact, this latter is usually the most satisfactory method of conducting the experiment.

If the ignited or calcined substance be soluble in water, it can be removed from the crucible by that menstruum, employing heat if required; if not, any suitable acid may be used.

If the substance to be calcined decrepitates on heating, it must be previously pulverised and heated slowly and gradually in a well-covered crucible.

Certain substances, as carbonate of lead, undergo a material alteration by contact with the gases given off during the combustion of the fuel in the heating furnace; others, such as carbonaceous matters, are consumed by the introduction of atmospheric air. All such substances must be calcined in a closely-covered crucible placed in a second crucible (also covered) for further protection.

In some rare cases, however, these precautions are not sufficient. In, such, either a weighed porcelain or German glass retort must be employed.

Sometimes earthenware crucibles lined with charcoal are employed in calcination; for even if the substance be fusible it may generally be collected and weighed without loss, as very few bodies either penetrate into or adhere to a charcoal lining. In this way grey cobalt and other arsenio-sulphides are calcined at a high temperature to expel the greatest possible amount of arsenic and sulphur.

The selection and proper management of crucibles will be given in the next chapter.