OXIDES OF IRON. - The red and black oxides of iron, and mixtures of them, are prepared by manufacturing chemists at Liverpool, Sheffield, Derby and elsewhere, as polishing powders, commercially known as crocus, rouge, red stuff, colcothar of vitriol, etc, and the same substances are also employed as pigments, under the names of red-brown, purple-brown, etc. The ordinary manufacture of crocus will be first noticed, and then the more exact method, required in the higher branches of scientific art, in order completely to avoid the accidental admixtures of silex and other impurities. As however these several matters have been elsewhere described with great exactness, it is conceived best to quote these passages, and it is to be observed that articles 1, 4, 6 and 7 are literal extracts from Mr. Thomas Gill's paper on the preparation of the metallic oxides, contained in Tech. Repos. vol. 1, pages 431-5. 1. - Crocus and Rouge. - "These articles are manufactured at Liverpool," said the late Mr. Samuel Varley, "by persons who make it their sole occupation, in the following manner. They take crystals of sulphate of iron, (green vitriol or copperas,) immediately from the crystallising vessels, in the copperas works there, so as to have them as clean as possible; and instantly put them into crucibles or cast iron pots, and expose them to heat, without suffering the smallest particles of dust to get in, which would have a tendency to scratch the articles to be polished. Those portions which are least calcined, and are of a scarlet colour, are fit to make rouge for polishing gold or silver; whilst those which are more calcined, or have become red, purple, or bluish purple, form crocus fit for polishing brass or steel. Of these, the bluish-purpled coloured part are the hardest, and are found nearest to the bottom of the vessels, and consequently have been exposed to the greatest degree of heat."

2. - Mr. Andrew Ross's Mode of Preparing Oxides of Iron. - "Dissolve crystals of sulphate of iron in water; filter the solution, to separate some particles of silex which are generally present and sometimes are abundant; then precipitate from this filtered solution the protoxide of iron by the addition of a saturated solution of soda, which must also be filtered. This gray oxide is to be repeatedly washed and then dried; put it in this state into a crucible, and very gradually raise it to a dull red heat; then pour it into a clean metal or earthen dish, and while cooling it will absorb oxygen from the atmosphere, and acquire a beautiful dark-red colour. In this state it is fit for polishing the softer metals, as silver and gold, but will scarcely make any impression on hardened steel or glass. For these latter purposes, I discovered that it is the black oxide that effected the polish, (and this gives to the red oxide a purple hue, which is used as the criterion of its cutting quality in ordinary,) therefore, for polishing the harder materials, the oxide must be heated to a bright red, and kept in that state until a sufficient quantity of it is converted into black oxide to give the mass a deep purple hue when exposed to the atmosphere. I have converted the whole into black oxide; but this is liable to scratch, and does not work so pleasantly as when mixed with the softer material. The powder must now be levigated with a soft wrought iron spatula, upon a soft iron slab, and afterwards washed in a very weak solution of gum-arabic as recommended by Dr. Green in his paper on Specula. The oxide prepared in this manner is almost impalpable, and free from all extraneous matter, and has the requisite quality in an eminent degree for polishing steel, glass, the softer gems, etc. See Emery, article 4.

3. - Lord Rosse's Mode of Preparing The Peroxide of Iron. - "I prepare the peroxide of iron by precipitation with water of ammonia from a pure dilute solution of sulphate of iron; the precipitate is washed, pressed in a screw press till nearly dry, and exposed to a heat which in the dark appears a dull low red. The only points of importance are, that the sulphate of iron should be pure, that the water of ammonia should be decidedly in excess, and that the heat should not exceed that I have described. The colour will be a bright crimson inclining to yellow. I have tried both potash and soda pure instead of water of ammonia, but after washing with some degree of care, a trace of the alkali still remained, and the peroxide was of an ochrey colour till overheated, and did not polish properly." See Phil. Trans., 1840, p. 521.

4. - Jewellers' Rouge___"Is prepared by persons in this metropolis, by decomposing sulphate of iron with potash; well washing the yellow oxide of iron, to free it from the sulphate of potash; and slightly calcining it, till it acquires a scarlet colour."

5. - Specular Iron Ore when finely pulverized and washed, makes a polishing powder which is greatly recommended by Mr. Heath for razor strops and other uses. It closely resembles both in appearance and effect the crocus artificially prepared from the sulphate of iron.

6. - Artificial Specular Iron Ore. - "This is made in the following manner. Equal parts of sulphate of iron and hydrochlorate of soda, (common salt,) are to be well mixed, by rubbing them together in a mortar: the mixture is then to be put into a shallow cupel or crucible, and exposed to a red-heat: a considerable quantity of vapour will be disengaged, and the matter will run into fusion. When vapours no longer arise, remove the vessel, and let it cool. "The mass will be of a violet-brown colour, covered with extremely brilliant scales resembling mica, and perfectly like the specular iron-ore. This mass must be dissolved in water; as well to separate the sulphate of soda which is formed by the decomposition of the two salts employed, as to wash over the lighter particles of uncrystallized oxide, which forms an excellent polishing powder.

"The fire must not be continued too long, nor be too violent; for then the powder would become black, extremely hard, and produce no good effect. The artificial specular iron ore is the more preferred, the nearer it approximates to the violet colour.

"The micaceous scales which subside after the washing over of the powdery part, afford an excellent material for razor strops, when applied to the strop with a little grease previously rubbed over it; as we can vouch, from our own experience in the use of it, for several months past."

It has been suggested to the author by an experienced chemist, that the atomic proportions of the sulphate of iron and common salt, should be taken for the last process, and when it is considered that, as noticed by the Earl of Rosse, the present limit of perfection in the polishing of specula, depends mainly on the fineness and efficiency of the polishing material, it becomes evident that the subject demands every care in its investigation, and which may apologize for the length of the foregoing articles. 7. - An improved Tripoli, for Polishing Gold and Silver. - "The basis of this excellent Tripoli consists of a mineral substance, abundantly found in the coal and iron mines of Staffordshire, etc. etc.; known by the name of clunch, or curl stone. It had formerly been employed for no other purpose than as a material for mending the roads. It is a compound of iron, alumine, lime, and silex." - Mr. Gill proposed this application of clunch from the external and chemical resemblance it bears to Septaria - the well-known basis of the Roman Cement, the employment of which in polishing he had previously advocated in the Annals of Philosophy. He goes on to say - "The polishing effects of the calcined and pulverized clunch are however still superior to that of the Septaria, when prepared in a similar manner; and are, indeed, in point of quickness of action in producing the polish, and in the beautiful black lustre which it gives to the gold or silver, far beyond any thing I have ever met with."