moistened by the well-stirred liquid; as soon as the paper is only faintly reddened by the liquid the ammonia is added very carefully, stirring well after the addition of each drop, and trying the action of the liquid by touching the wet glass rod against first blue and then red litmus-paper, waiting a short time to give the liquid time to act on the paper. When the colours of both blue and red paper remain unaffected, the process of "neutralisation" is completed.

It is well to add dilute ammonia towards the end of the process; this is prepared by mixing a little ammonia with three or four times as much distilled water in a test-tube.

The process of neutralising may often be simplified when the liquid is clear and colourless, by dropping into it a small piece of blue litmus-paper, and after each addition of ammonia noting its colour; the condition of the liquid should, however, even in this case be proved towards the end by moistening both red and blue paper.

The process of neutralising an alkali with an acid is precisely similar to that of neutralising an acid with an alkali, save that red litmus is at first used instead of blue.

35a. It will be readily understood from what has been said above, that if an alkaline or neutral liquid is to be "acidified,” acid is added gradually as was described in the preceding experiment, until a drop of the well-mixed liquid reddens blue litmus. If an acid or neutral liquid has to be made alkaline, the alkali is added until a drop of the liquid turns red litmus blue. These processes are often termed adding acid or alkali "in excess.'

35b. Yellow turmeric paper is occasionally employed instead of red litmus to test for alkalis, which turn it reddish-brown; it is unaffected by acids, with the exception of boracic acid. The most important use for turmeric paper is to test for boracic acid, which, if dried on the paper at a gentle heat, turns it orange-red.

EXP. 55. This may be shown by stirring a little borax upon a watch-glass with dilute hydrochloric acid until it is dissolved, moistening the lower part of a slip of turmeric paper with this liquid, and drying it by holding it at some

distance above the flame, or better by placing it in a steamoven; the portion of the slip which was moistened will now appear reddish-brown, the colour being well seen by contrast with the light yellow upper portion of the paper.

FRACTIONAL SOLUTION, CRYSTALLISATION

AND DISTILLATION.

1. Fractional Solution.-Since different substances show differences in solubility in water and acids, it is often possible by means of solvents to separate the constituents of a mixture. This process is termed separation by "fractional solution." Illustrate this by making a mixture of powdered ferrous sulphate (FeSO4.7H2O) and ferric oxide (FeO3); on warming a portion of this with water, FeSO4.7H2O alone dissolves, leaving Fe,O,: if another portion is warmed with HCl, both substances dissolve.

3

2. Fractional Crystallisation.—If a solution of two different salts be boiled down in an evaporating basin, one salt will usually furnish crystals before the other does, and by this process of "fractional crystallisation" a separation of the salts may often be effected. Boil down a solution of sodium chloride (NaCl) and nitre (KNO3): NaCl will crystallise out of the boiling liquid in hollow cubes, and on cooling prisms of nitre will separate.

3. Fractional Distillation.-Liquids which boil at different temperatures may often be separated by distillation (p. 24); that part of the mixture which first distils over consisting chiefly of the liquid with the lowest boiling-point, and the successive portions of the distillate becoming richer in the liquids of higher boiling points. The process of" fractional distillation" may be illustrated by distilling a mixture of alcohol and water; since alcohol boils at a lower temperature than water, the first part of the distillate contains the greater part of the alcohol, as is shown by its burning and by its dissolving camphor or iodine in quantity. The last portion of the distillate does not give these tests for alcohol, and is proved to be water by turning dry CuSO, blue and dissolving it.

SECTION IV.

ANALYTICAL REACTIONS.

Course of Practical Analysis to be pursued by the Student.The text is so arranged that two main courses may be framed from it in this and the following sections. The one is suited to senior students and dispenses with the training in the detection of simple substances, which, although useful as a stepping-stone to the full analytical method, is in itself of only artificial importance. The other course is suited to junior students, and forms an easy approach to the more difficult complete methods of analysis: it teaches the plan of analysing simple substances which may contain one acid and one metal. This course is therefore also suited to those who are learning analytical chemistry with the view of preparing themselves for examinations in which the candidate is not required to detect more than one acid and one metal. These courses are described below as the "Senior Course" and "Junior Course."

The Senior Course consists in working through the analytical reactions in Section IV.; and as those for each group are completed, several substances whose composition is unknown to the students are tested for one member only by the Table of Differences at the end of the Group; as soon as these are detected with readiness, substances which may contain two or more of the members mixed together are analysed by the Group-table in Section VI.: a reference to the Table of Differences will show at a glance on what principles these Group-tables are founded, and this is further explained by a statement following the Table of Differences. This plan of working with each Group-table before going on to the reactions for the next Group is to be recommended, since, whilst the differences upon which the separation and special tests of

the elements in the Groups are fresh in the student's memory, they are practically applied. The Analytical Tables are thus gradually rendered intelligible one after another, whilst the student is proceeding through the reactions. The book is so arranged, however, that all the Analytical Tables may be left until the reactions for all the groups have been finished. When all the groups have been worked through in this manner, the analysis of substances is commenced which may contain any members of any of the different analytical groups; the directions for the systematic method of analysis required for this purpose being contained in Section VI.

The Junior Course. Since this is intended to teach only the detection of one metal and one acid; the student whilst working through Section IV., omits all those paragraphs marked "s" after the number, relating to the separation and detection of the mixed members of a group; and after finishing the reactions for each group, he proceeds to detect the members occurring singly in several substances of unknown composition by the Table of Differences and by reference to the reactions. After working in this way through Section IV., he at once proceeds to analyse simple substances by Section V.

As stated above, this Junior Course may occasionally be made to precede the Senior with advantage—that is to say, the student, after working through the Junior Course and learning how to detect simple substances, may analyse mixtures containing two or more members of each group by the directions given at the end of each of the groups in Section IV., and may then proceed to the full method of analysis contained in Section VI.

INTRODUCTORY REMARKS ON ANALYTICAL CHEMISTRY.

The science of Chemistry reveals to us the fact that every substance on this earth, and probably in the universe, consists either of one kind of matter only, or of two or more different kinds, which we cannot further separate. Those substances which contain only one kind of matter, are called "chemical elements;" their number at present amounts to a little over sixty. A list of them is given in paragraph 558 at the end of the book.

It is not often that these elements occur singly; two or more of them are commonly found together either in a state of mere "mixture," or united in a much more intimate manner by the force of "chemical affinity" to form a "chemical compound."

Analytical chemistry or analysis (literally meaning an unlovsing or separation), teaches us how to discover what element, elements, or sometimes what groups of elements, any substance of unknown composition contains: it also enables us to ascertain whether any particular element or group of elements is present in a given substance. In order to become a thorough analyst, it is necessary to be intimately acquainted with the properties of every element and of the compounds. it forms with other elements, and therefore efficiency in analysis depends in a large measure upon the knowledge of what is usually termed "Theoretical," or more appropriately perhaps "Descriptive, Chemistry." But although we recognise a substance by its properties, still a knowledge of all its properties is not requisite to enable us to detect its presence: for ordinary analytical purposes certain marked properties of each element or group of elements are selected, and by these properties, called "tests" or "reactions," we detect its presence.

The "tests or "reactions" most frequently selected are the following:-1st, The behaviour of a substance when heated under various conditions; and 2nd, Its behaviour