579. We shall in another chapter explain how atomic and molecular weights are determined.

435. Define

'molecule.'

Examination Questions.

element,'

'compound,' 'equivalent,' 'atom,' and

436. How do you distinguish chemical affinity from the other molecular forces ?

437. State the relation that generally exists between the sp. gr. of an elementary gas or vapour compared with hydrogen as unity, and its atomic weight.

438. How do you deduce the molecular weight of a gaseous or vaporisable compound from its sp. gr.? Give examples.

BASIC SULPHIDES

CHAPTER XI.

EXERCISES

QUESTIONS

CHEMICAL NOMENCLATURE AND NOTATION-EXERCISES-BASIC SUBSTANCES-TABLE OF THE MORE COMMONLY OCCURRING BASESTHE METHODS ADOPTED FOR PREPARING THE BASES-TABLE SHOWING THE SOLUBILITY OF THE BASES IN WATER AND THE ALKALIES-EXERCISES QUESTIONS.

580. NOMENCLATURE is the spoken language of chemistry, as notation is the symbolic written language of the science; and we have to explain the rules for naming and symbolising chemical compounds. We will commence with binary compounds, that is compounds which are composed of two elements only. The name of one of the elements in these compounds ends in ide, as oxide, sulphide, hydride, &c. If the compound consists of a metal and non-metal, it is the name of the non-metal that terminates in ide; and the name of the metal is now most frequently placed first, as potassium oxide, sodium sulphide, magnesium chloride, &c. When the two elements in combination are both non-metals, there is no exact rule as to which of the two names shall end in ide, but whichever name is made to end in ide it is placed last, as in the previous examples-thus, phosphorus chloride. Formerly the name of the non-metal was placed first. Examples: oxide of zinc, chloride of sodium, iodide of lead, &c.; and this method of naming binary compounds is not yet obsolete.

581. Many of the elements unite, as we are already aware, in more atomic proportions than one, forming several distinct

chemical compounds; prefixes have therefore to be used to indicate the respective compounds. When one of the elements unites in six different proportions with the other element, say in the proportion of the numbers 1, 2, 3, 4, 5, and 6, the prefixes are mon, di, tri, tetra, penta, hex; in place of these Greek prefixes, the Latin prefixes uni, bi, ter, quadro, quinque, sex are often used; thus we can say potassium disulphide, or potassium bisulphide, both are equally correct; but the Greek prefixes are more generally employed at the present time. When the atomic relation between the two compounds is in the proportion of 2 to 3, the Latin prefix sesqui, meaning one and a half, is employed. The highest oxides, chlorides, sulphides, &c. are sometimes termed peroxides, perchlorides, &c. When the two elements are combined together in the proportion of one atom to one atom the prefix is frequently dispensed with, as, for instance, potassium chloride (KCl).

582. The student is already aware that when two distinct series of compounds are formed by the same elements, the name of one of the elements in the lower compound ends in ous, and the name of the higher one in ic; and if one of the elements is a metal, it is the name of the metal that so terminates, thus ferrous chloride, ferric chloride. There is one other point we must draw the attention of the student to; it is this, that some chemists make the name of the metal in a compound terminate in ic, instead of in um, as potassic oxide, zincic chloride, plumbic iodide, &c.

Exercises on Nomenclature.

439. Name the following compounds :

2.

CaF KS4 Cr2O. NiCl2. KO. HgCl. HgCl. CoS. AuCl ̧. PtCl,.

583. We will not proceed further at present with the nomenclature, but will commence with the notation. We already know that a small figure placed below the line multiplies only the element on the left-hand side of it, but this is not the case if the compound is enclosed in a bracket; for if enclosed in a bracket the numeral indicates the number of molecules of a compound; thus, (HCl), denotes 3 molecules of the compound, hydrogen chloride; the small figure

may be placed either above or below the line, not upon it. A large figure placed upon the line denotes the number of molecules of a compound on the right of it; thus, 2Cr2O3 denotes 2 molecules of chromium sesquioxide; the figure on the line multiplies all that follow, up to a comma, full-stop, or plus sign, not beyond these signs. When a compound is composed of two other compounds, it is usual to separate the molecules by a comma. Example: 2 KCl,PtCl. When a period is placed between compounds it indicates that they are in less intimate union than if they had been separated by a comma. 'For instance, in the formula for crystallised sulphate of magnesium and potassium, MgSO4, K2SO4.6H2O, the compound MgSO4 is supposed to be more intimately united with K2SO4 than with the 6H2O, which may be expelled by heat.' The plus sign is sometimes employed in place of the full-stop. As a figure upon the line multiplies only up to a comma, full-stop, or plus sign, it is necessary, when we have to express more than one molecule of a substance, in the formula of which one or more of these signs have to be employed, to enclose the formula in a bracket, and place the number outside it, either before or after. Examples: 2(MgSO4, K2SO4.6H2O); (ZnCl2, 2 NaCl)2.

Exercises on Notation.

440. Explain the following formulæ, the term formula being applied to a group of two or more symbols; the student is not expected to give the names of the compounds:

Ba(NO3)2. ZnCl2, 2KCl. 2(KI, HgI2), 3(H2O), 3(FeSo,,7H2O). AIF3,3NaF. Al(PO1).

584. The student has already been informed that there are four different classes of compound substances, viz., bases, acids, salts, and indifferent bodies. We have now to make the student more fully acquainted with bases, acids, and salts, and we shall commence with the basic bodies.

Bases are bodies which unite with acids, and form with them the class of compounds termed salts. There are three classes of compounds to which the term base is applied. These

are

Ist. Compounds composed of metals, or compound radicals playing the part of metals and oxygen. Example :

NEIL Fe' The bases of this class are

me Camporrmos composed of metals, or compound radimus daring de put of meals and the univalent radical hydroxyl 三. Emotie · NLER NH,HO.Zn H2O, Fe"2H6O6. The cases of ms ass e frequently called hydrated bases. Compounds composed of hydrogen and nitrogen, and

sydrogen and postors Exmple: NH,

$. The buses which are stible in water restore the blue colder to domes vach has been reddened by an acid.

The flowing salt of the more important bases belonging no the 1st and and casses, and the student, before proceeding farben must coast to pay the names and formulæ of Dese buses. If the stodem does not follow my advice in com

ng to memory the names and symbols of the elements, and the names and formale of these bases and the other typical compounds which follow, be will encounter difficulties that he would not encounter if he had followed the advice given him.

586. The names placed in the brackets, in the following Est. are the commercial and common names of the substances, as for example. caustic potash, caustic soda, quicklime, slaked lime, &c.

587. The student will not fall to notice that the hydrates of a few of the oxides are not given in the table: whether the oxides of these metals can form hydrates or not is uncertain ; all that can be said is, they have not yet been obtained. The term hydrated was applied to the bases which contain hydrogen as well as oxygen when it was thought the whole of the hydrogen was united with a portion of the oxygen in the proportion to form water. The hydrated bases from this point of view are compounds of the oxides of the metals and water, and their formulæ, in accordance with this view, are thus expressed: Potassium hydrate, K,O,H2O, Barium hydrate, BaO,H,0, Stannic hydrate, SnO2,2H2O, Ferric hydrate, Fe2O3,3H2O, &c.; and this mode of representing this class of bases is still adopted by some chemical writers. Instead of the formulæ given in the table for this class of bases some chemical writers represent them thus :-KHO, Ba(OH)2,Sn(OH)4,Fe(OH)6, &c. ; but, as the student will not fail to see, these formulæ, and those given

Zinc oxide.

Zinc hydrate

Plumbic hydrate

PbH2O2

SnHO,

PtH,O1