fore the taste, colour, and other general properties of the solid, and yielding again the solid substance when the solvent is removed by "evaporation." Solution of sodium chloride in water is an example of a "simple solution."

"Chemical solution," on the other hand, is always attended by a chemical change in the substance to be dissolved; and since the solution therefore contains a substance differing in composition from the undissolved solid, the properties of the solution are usually not the same of those of the undissolved solid, and on removing the solvent by evaporation the original substance is not obtained.

In dissolving solid substances in liquids, test-tubes, porcelain basins, or small glass flasks are generally employed.

Exp. 33.—Place a piece of potassium nitrate (nitre) in a small clean beaker, fill the beaker one-third with water,* and stir the solid about in the water with a glass rod the potassium nitrate will slowly dissolve in the water; if the liquid be heated in the Bunsen flame the solution will be much more rapidly effected. Powder another piece of potassium nitrate by crushing it and then rubbing it in a mortar with the pestle; place this powder in a test-tube, pour in water, and heat the bottom of the test-tube; the potassium nitrate will dissolve much more rapidly than before, showing that solution is accelerated by powdering the solid and employing heat. Keep these solutions.

EXP. 34.-Powder a little copper sulphate (blue vitriol) in a mortar, place it in a small porcelain dish, half fill the dish with water, and heat it on a tripod or retort stand. The blue copper sulphate dissolves, yielding a blue solution. Keep this solution.

These two experiments are both examples of "simple solution." Exp. 33 shows that a colourless solid gives a colourless solution. Exp. 34 proves that a coloured solid gives a coloured solution. This is generally true, and hence we can often infer the presence or absence of a coloured substance in a solution by merely noting the colour of the liquid. More

* Here and in all subsequent cases water must be understood to signify distilled water.

over, if a drop of the potassium nitrate solution be tasted, it will be found to possess the same taste as the solid; chemists occasionally rely upon taste, as upon colour, when examining "simple solutions."

EXP. 35.-Place in a test-tube a few small pieces of calcium carbonate (marble or chalk), pour upon them a little water and heat the tube; the calcium carbonate will be found to be "insoluble" in the water. Add to the water

some hydrochloric acid : "effervescence," or escape of numerous small bubbles of gas, will occur; the pieces of calcium carbonate will meanwhile slowly diminish in size, and will at last entirely disappear in the liquid if sufficient acid is added.

EXP, 36.-Place in a test-tube a few small pieces of copper, on warming these with a little water they remain undissolved, but on adding to the water some nitric acid and heating, the copper slowly dissolves giving off red fumes, and will be entirely dissolved if sufficient nitric acid is employed.

Experiments 35 and 36 are examples of "chemical solution;" the calcium carbonate is changed by the hydrochloric acid into calcium chloride, and this substance, not the calcium carbonate, remains in solution. The copper is changed into copper nitrate, which is then dissolved by the water. It will be noticed that in both these cases a gas is given off; this is a very usual, but not a universal, effect during "chemical solution:" the distinguishing fact is that the solid substance which is to be dissolved has undergone a chemical change in the act of passing into solution.

21. Evaporation.-When it is wished to obtain a sub. stance, which is dissolved in a liquid, in the solid condition, the liquid is boiled away as vapour, or "evaporated;" the solid substance is then left behind in the vessel.

EXP. 37.-Pour the potassium nitrate solution from Exp. 33 into a porcelain evaporating basin, place the basin on a tripod-stand and boil it over the Bunsen flame until the water has been nearly boiled away, then make the flame smaller and heat until the water has quite disappeared: the solid potassium nitrate is left in the dish.

In evaporating a solution a small flame or a rose-burner

should always be used towards the end of the process, and the flame should be moved about if necessary, else some of the substance will spirt out of the dish.

Often a part only of the liquid is evaporated for the purpose of "concentrating" the solution (i.e., making it stronger by removing a portion of the solvent). A solution, when sufficiently concentrated, will often form "crystals" of the dissolved substance if allowed to stand till cold.

EXP. 37 a.-Concentrate the copper sulphate solution made in Exp. 34, and allow it to cool; if sufficient water has been evaporated it will form crystals. Keep the copper sulphate in the dish.

22. Precipitation.-Two perfectly clear and transparent solutions, on being mixed together, often become more or less turbid and opaque, owing to a solid insoluble substance being formed in the liquid. A solid substance so formed in a liquid is called a "precipitate."

EXP. 38. Pour into a test-tube some barium chloride solution, and then some ammonium carbonate solution a white precipitate is formed owing to the production of insoluble solid barium carbonate. Keep this test-tube with the precipitate.

In the above instance barium chloride and ammonium carbonate separately dissolve easily in water, but if their solutions are mixed they yield by a chemical change two different substances-ammonium chloride and barium carbonate; the former of these dissolves in water, but the latter (like common marble) is insoluble, and therefore remains as a fine powder in the liquid.

Substances are very frequently removed or separated in analysis by thus causing them to form insoluble compounds or precipitates with other substances which are added for the purpose.

In producing a precipitate care must be taken that the two solutions are well mixed; this mixture may be effected by closing the mouth of the test-tube with the thumb and several times inverting it, or often by simply warming the bottom of the test-tube in the flame; mixture may also be

effected by stirring with a glass rod or by pouring the liquid from one vessel to another: the last three methods are to be used when the liquid present is corrosive, and therefore must not be allowed to touch the skin. A precipitate which does not form readily is often caused to appear more rapidly on thoroughly mixing or agitating the liquid by the above means, or by heating it.

Precipitates produced with different substances differ much in appearance and properties; hence we frequently form a precipitate to show the presence of a substance or to separate one substance from another. Substances thus added to produce precipitates are called "reagents."

The appearance of a precipitate is usually described by its colour and its condition: it is "flocculent" if it forms in flock-like masses, "crystalline" if in small particles which are seen to be crystals under a lens or microscope, "gelatinous" if jelly-like, &c. A slight precipitate causes only a "turbidity" in a liquid.

The colour of a precipitate or liquid is often invisible or falsified by gaslight; if the colour is to be seen at night, it should be examined with the light produced by a piece of burning magnesium ribbon.

Occasionally solid substances are used to produce precipitates thus one metal is not unfrequently precipitated from its solution by immersing in the liquid another metal.

EXP. 39.-Dip a clean penknife blade into some of the copper sulphate solution left from Exp. 37 a, to which a few drops of sulphuric acid have been added; after a short time the metal copper is precipitated from its solution. and covers the iron as a red film.

23. Filtration and Decantation.-It is frequently necessary to separate a precipitate from the liquid in which it is suspended. This is effected either by filtration or decantation.

FIG. 24.

23 a. Filtration is performed by pouring the liquid containing the precipitate upon some porous paper; the liquid itself runs through the pores of the paper, but the solid particles of the precipitate cannot pass through, they

remain on the surface of the paper. The paper employed is called "filtering paper," and the liquid which runs through is termed the "filtrate." A filtrate may frequently be coloured. by some substance dissolved in it, but it must always be perfectly clear and free from turbidity caused by solid particles suspended in it.

EXP. 40.-Measure a glass funnel from its shoulder to its edge along the sloping side (fig. 24), cut a square of filter

FIG. 25.

a

paper whose edge is rather less than double this length, fold it across first along the dotted line (fig. 25 a), then again as shown by the dotted line in fig. 25b; this gives a square (fig. 26 c) which at one angle has four free corners, these are removed by cutting with a pair of scissors along the curved dotted line shown in fig 25 c; the "filter" is now made and merely requires to be opened; by separating the edges, formed by cutting off the corners, so that they form a circle, three remaining on one side of the circle and one on the other, a little closed pointed paper bag is formed (fig. 25 d); this is gently pressed with dry fingers into the dry funnel, and the folding altered if necessary till it fits tightly into the glass; it is then moistened all over with a little water from the wash-bottle, and is ready for use.

The preliminary moistening of the filter-paper must not be neglected, since if the liquid and precipitate are poured upon a dry filter, some particles of the precipitate get between