speedily become acquainted; it is a substance of great importance analytically, and is, moreover, a remarkable instance of the manner in which the basic element hydrogen overcomes the feebly acid properties of nitrogen, and thus confers on their combination a basic character. The acid compounds are far more numerous and important, and are either produced by the union of the acid elements among themselves, or by their combination with basic elements in such proportion as to allow of the preponderance of their own properties over those of the basic elements. It is singular also that many of these acid compounds result from the combination of the two acid elements, sulphur and oxygen, with the remaining bodies of their own class; the composition of a few such compounds is given below, with the names which, by long usage, have become attached to them and cannot be desirably removed, although they must be regarded as wholly apart from any theoretical considerations touching the nature and constitution of the bodies which they designate; the examples are given as existing in saline combination with hydrogen :

With regard to the nomenclature of these compounds, it may be as well remembered that (with one or two exceptions, as in the case of sulphocyanogen (CyS)), when the compound acid portion of a binary combination contains no oxygen or sulphur, the termination of its own name is altered into -ide when we wish to designate any compound which it forms with a basic element, thus:

An exception is here made when these acid bodies unite with the basic element hydrogen; and though it is chemically correct to call such a compound as HCl " chloride of hydrogen," yet conventionality demands that it be also termed "hydrochloric or chlorhydric acid ;" and thus with all such compounds.

Now, if oxygen be present in the acid compound, a totally distinct nomenclature is adopted; these bodies combined with hydrogen constitute a very large section of the so-called acids, and receive either the termination -ic, as nitric acid (HNO,), or -ous, as hypochlorous acid (HClO). When the hydrogen in one. of these groups has been replaced by another basic element, the designation that previously terminated in -ic now ends in -ate, and that in -ous in -ite; as nitrate of potassium (KNO,), and hypochlorite of calcium (CaClO). From the following Table of the hydrogen and potassium salts of some compound acid-radicals which contain sulphur and oxygen, the student will perceive the usage and changes of these terms:

Here is a similar list, containing the hydrogen and potassium salts of a series of compound acid-radicals formed by the union of

It will have been observed, that in the Table of acids given at page 45 there are three classes of those bodies, distinguished by containing a different amount of hydrogen, that is, of basic element: these classes are the groups of monobasic, bibasic, and tribasic acids which contain acid-radicals requiring these different amounts of basic element to satisfy their respective combining powers; and if the student will refer to what was said regarding the three series of acid elements at page 38, he will at once perceive that these three series of compound acid-radicals accu

SILVER SALTS OF

rately correspond with them. This relation may be exhibited

thus:

The difference between a ferrous and a ferric, between a cuprous and a cupric salt of an elementary acid-radical, has been already pointed out; it now only remains to transfer this distinction to the corresponding salts of the compound acid-radicals. We will take potassium, iron, and bismuth as the basic elements in our salts; and, for the compound acid-radicals, those existing in the nitric, sulphuric, and phosphoric acids. Now, potassium is monatomic, i. e. K1=H, ; and iron is sometimes sesquiatomic, i.e. Fe, H1, or Fe,=H,; while bismuth is triatomic, i. e. Bi,=H,. Taking these values, we arrive at the following formula for the different salts of the same acids :

=

Here it is seen how the formula of the same description of salt varies, not only with the saturating power of the acid body, but also with the atomic function of the basic constituent.

A word in explanation of a term in general use, the term "basic salt: "—when an absolutely perfect double decomposition does not occur, the precipitate produced, instead of being a pure salt, is a compound one; this may be best illustrated by a hypo

thetical example, where M stands for a basic radical, such as one of the metals

20 M C1+19 KHO=19(MHO), MC1+19 KC1.

Basic salt.

With regard to solvents;—the action of these agents is exceedingly obscure; they appear to fulfil an office intermediate between chemical union and mechanical mixture. The student had better regard them simply as agents for presenting bodies in a liquid form, and as rarely participating in the chemical actions which take place in their very midst of course they do occasionally exert very powerful chemical actions; but these instances can be readily discriminated. The more stable the equilibrium of a body, the less chemical influence is it likely to exert; and thus water is far less likely to interfere than the acids, and is for this reason always employed wherever it is practicable, as the solvent to which the least objection can be raised.

CHAPTER V.

OF REAGENTS.

REAGENTS are those substances which by admixture we bring to act upon the bodies we desire to analyse, in such a manner as to produce certain phenomena which shall prove indubitably the presence of the substance sought for. If in such circumstances the expected effect is not produced, we must infer the absence of the object of our search.

It is obviously, therefore, of the greatest importance that the reagents which are employed in chemical analysis should, if not absolutely pure, at least be free from substances which would interfere with the indications which they are employed to give. The amount and the nature of the impurities which they contain should be accurately known.

Most of the reagents in common use are met with in commerce of sufficient purity to allow of their application in all but the more delicate operations of analysis. For certain special cases, however, as in legal investigations for the purpose of ascertaining the presence or absence of poison, too much care cannot be bestowed upon the preparation of absolutely pure reagents; and in such instances it is generally necessary for the chemist to ensure the purity of

the materials he employs by preparing, or at least scrupulously purifying them himself.

The ordinary methods of purification are briefly these: Sublimation, Crystallization, Precipitation, and Distillation.

SUBLIMATION can only be resorted to when it is desired to free a body solid at ordinary temperatures, but volatile at a higher degree of heat, from nonvolatile impurities; iodine is usually purified in this way: many mercury and ammonium salts might also be thus freed from accompanying foreign matters which were not volatile. In the sublimation of many substances it is simply necessary to place the substance in a dish over which is fixed a tall beaker or bell-jar, which may be luted or otherwise attached to the dish below; if the dish containing the substance is now heated, the volatile substance (iodine, oxalic acid, &c., as the case may be) rises in vapour,-speedily however condensing on the cold surface of the receiver above.

CRYSTALLIZATION is perhaps the most commonly adopted of all methods of purification. In general, substances dissolve more abundantly in hot than in cold solvents. If then at some given temperature a menstruum, as water, alcohol or æther, be saturated with any solid body (i.e. supplied with it until no more is dissolved), the solution perfectly transparent at that degree of heat, will, as soon as the temperature diminishes, deposit some of the dissolved matter in the solid form. In the same manner let a given bulk of a saturated solution of almost any substance be taken, and without increasing its temperature, let its volume be diminished to one half (as by slow spontaneous evaporation), then a considerable portion of the solid matter before held in solution will be found to have separated in the form of crystals. Upon these two facts is based the method of purification by crystallization.

Salts differ much as to their solubility in the same menstruum; and by taking advantage of the facts which experimenters have accumulated upon this point, we can separate different compounds with considerable accuracy by evaporating the solution containing the mixture, we will suppose of two salts, to that degree of concentration at which one of them will almost entirely separate, while the other will remain almost as wholly in solution. By the repetition of this process upon the already partially purified product of the first crystallization, greater purity is attained. But with many articles of commerce one carefully conducted crystallization is often found sufficient. The technical name for the liquid remaining after the separation of the crystals is "mother-liquor," while the successive products of the crystallizing process are spoken of as "crops" of crystals.

PRECIPITATION is frequently employed when the substance constituting the impurity is known to form an insoluble compound on the addition of some reagent the introduction of which does not interfere with the efficiency of the reagent we seek to purify. Let it be supposed that chloride of ammonium (NH, Cl) is the substance with which we wish to deal. Now the impurity which this salt commonly contains is chloride of iron (FeCl); to separate this the substance is dissolved in water, and a few drops of sulphide

D