The varieties of writing-fluids that have been devised and introduced are almost innumerable, but for practical purposes the inks in common use may be divided into three classes, viz: 1. Those which consist of a powder mechanically divided and suspended in water by means of mucilage. 2. Those which consist of chemical precipitates held in suspension in the same way. 3. Those which consist of a true solution of some coloring matter, such as aniline or carmine. Of the first class, Indian or China ink is the great type. It consists of carbon in the form of very fine lamp-black, ground to a state of impalpable fineness in water, and mixed with some pure form of gelatine. Its use is wholly restricted to draughtsmen, who prefer it for several reasons. In the first place, it gives the finest and clearest black of any ink known; second, it is unchangeable; and in the third place, it does not corrode the fine and expensive steel instruments with which it is used. A really good article of Indian ink is somewhat difficult to find. Much of the ink in market is gritty, and instead of being a fine jet black, it is of a blueish-gray color. Moreover, notwithstanding all the grinding that the artist can give it, the particles are always coarse, and it does not readily sink into the paper. With such ink it is difficult to draw fine, clear, black lines, and utterly impossible to produce a soft mellow tint in shading. It is probable that the quality of the ink depends not only upon the materials from which it is made, but upon the method pursued in its manufacture, and in regard to both these points we are as yet wholly in the dark. When good Indian ink is wanted, therefore, the only method of securing it is to test carefully the various samples, until we get a good one, and then secure a supply that will last indefinitely. Fortunately the last is not a difficult thing to do, when we have found a sample that suits us; for a single stick of Indian ink, if carefully used, will last many years, even in the hands of a professional draughtsman. Of late years a liquid Indian ink has been introduced, and has given good satisfaction, but it is scarce and expensive. Since the ordinary Indian ink is made up with a fine animal glue, instead of mucilage made of vegetable gum, it very soon decomposes when ground up with water. Hence it can not be kept in bottles like ordinary ink, but must be prepared fresh whenever it is needed. As an ink for ordinary writing it is worthless, for the simple reason that it does not flow well, though for purposes where an absolutely indelible ink is needed - as, for instance, in writing out deeds and records - nothing better can be obtained. When used for this purpose, the addition of a very small quantity of caustic alkali - or, what is better yet, of ox-gall - causes it to flow freely and to sink deeply into the paper or other material use*? to receive it, provided the latter be not too heavily sized. When properly applied, neither heat, moisture, acids, alkalies, nor chemicals of any kind, affect it; and it might therefore be properly used to write those records which are placed under the corner-stones of important buildings, and which are expected to endure for an indefinite period.

The second class of inks comprises all those black inks and writing fluids that are commonly employed for commercial correspondence and records. The different formulae for the preparation of ink that have been published, would fill a good sized volume; but most of the inks and writing fluids in market consist of a precipitate of gallate or tannate of 7'ron. held in suspension by means of mucilage. Since iron may be used in either one of two distinct conditions when it is employed for the manufacture of ink, it follows that two distinct kinds of ink may be made froni it. In one of these the iron is fully oxidated, and the ink is of a deep jet black. The precipitate of iron which exists in such ink seems to assume a coarse and heavy form, with a strong tendency to sink to the bottom of the containing vessel. It therefore requires a large proportion of mucilage to keep the coloring matter in suspension. The advantage which it possesses, is, that the ink is, from the very first, of a deep black; but on the other hand, the objections are quite as important, and consist in the fact that it can not be made to flow freely, and that it does not sink well into the paper, and is consequently easily removed. On the other hand, ink made with salts in which the iron exists as protoxide, is always pale at first, but afterwards assumes a dark hue; it flows freely and sinks well into the fibre, so that it is difficult to remove marks made by it. This character it is apt to lose, however, when exposed to the air, as Ave shall note when speaking of the preservation of ink.

In some cases a compromise is made, and the ink is prepared from materials, part of which only are in a state of complete oxidation. An attempt is thus made to secure an ink, which, while black from the first, will flow freely and sink well into the paper, and some very good inks are thus compounded.

Most of the inks known as violet, mauve, blue, red, carmine, etc., consist of true chemical solutions, generally nowadays of aniline, though the finest red ink is still made from carmine dissolved in ammonia. From the fact that there is no solid material to be kept in supension, these inks do not require mucilage in their composition provided they are used on paper that has a good deal of size in it; they consequently flow freely, do not leave a heavy streak of liquid behind the pen, and the streak that they do leave sinks almost instantly into the paper and disappears. In using them, no blotter is required; and they are, therefore, great favorites with authors and those persons who pay less regard to the color of their writing than to the ease with which the work is done, and the clearness and unblotted appearance which it presents. But from the fact that no really good black ink of this class has yet been produced, they have not come into general use amongst book-keepers and commercial men, and it must be acknowledged that on the whole a good black ink gives a better appearance to a set of books than ink of any other color.

Ink used for copying letters by means of the press, requires to be thicker than that used for ordinary writing, and therefore it is less pleasant to use; but the great advantage which atfrjads the mechanical process of copying letters will always keep up the demand for it.

Sixch being the peculiar character of the inks in common use, it may be well to say a few words concerning the best methods of preserving them in good condition. The great enemies of all inks are evaporation, dust, and decomposition, and, in the case of iron inks, oxidation. The first difficulty can only be avoided by keeping the ink from exposure to the air, and this is best effected by adopting an inkstand in which the ink exposes a very small surface to the air. Many of the inkstands in use are made large at the base, for the purpose of rendering them difficult to overturn. In such stands the ink is spread out in a thin, wide layer, and not only evaporates rapidly, but where ordinary black ink is used, the iron oxidates, and the ink consequently deteriorates. A very common practice on the part of those who use ink, is to leave the mouth of the stand uncovered, in which case the ink becomes in a short time reduced to mud. All these difficulties may be in a measure avoided by using a heavy stand, having a small well or ink-holder, which should be kept well covered when not in use, and ought to be frequently cleaned, the old ink being thrown away. The supply of ink should be kept in a bottle, securely corked, and when the stand is filled, the new ink ought never to be poured into the old, as is generally done. Throw the old ink away; wash out the stand carefully, and fill it up with new fluid, and then you can enjoy the luxury of writing with ink that flows freely, and does not take half a minute to moisten the paper at each stroke that you attempt to make. To keep ink in good order, the stand should be washed out every two or three weeks.