20

PROPERTIES OF WATER.

and -1.3°, the direction of the variation not changing in the passage through the point of maximum density. At 0° the index is 1.333.

30. Pure water at 0°, a temperature always to be obtained by melting ice, is taken as a standard to which the weights of equal bulks of other substances, liquid or solid, are referred. In other words, the specific gravity of water is taken as 1; and in terms of this unit the specific gravities of all other liquid and solid substances are expressed. The specific gravity of gold, for example, is 19.3; that is to say, the weights of equal bulks of water and of gold are to one another as 1 to 19.3.

31. Water is also the standard of specific heat. By specific heats are meant the relative capacities for heat of the same weights of different substances, at the same temperature. For example, to raise 1 kilogramme of mercury from 0° to 1° requires only one-thirtieth of the quantity of heat necessary to raise 1 kilogramme of water from 0° to 1°. Water having been made the standard of specific heat, its capacity for heat is denoted by 1, and that of mercury will accordingly be 0.033. At the same temperature, and for equal weights, water has a greater capacity for heat than any solid or liquid known. Hence it results that the specific heats of all solid and liquid substances are expressed by fractions.

Water conducts heat very slowly; it may be boiled many minutes at the top of the test-tube, which is held all the while by the lower end, in the fingers, without inconvenience.

32. When exposed to a certain degree of cold, water crystallizes, with formation of ice, or snow, according to circumstances; and upon being heated sufficiently it is transformed into an invisible gas, called steam. Both these changes, however, are purely physical, and therefore do not fall within the province of this manual. The chemical composition of the water is the same, whether it be liquid, solid, or gaseous. The temperature at which ice melts is one of the fixed points of the centigrade thermometer, numbered 0°, and the temperature at which water boils, under a pressure of 76 c.m. of mercury, is the other fixed point, numbered 100°. Water evaporates at all temperatures, and is therefore a constant ingredient of the atmosphere. Even

ice slowly evaporates, at temperatures far below 0°, without first passing into the liquid condition.

In crystallizing, that is to say, in assuming the solid form, water increases in volume. The specific gravity of ice is only 0-916, which is equivalent to saying that, in the act of freezing, 916 c. c. (cubic centimetres) of water will be changed into a litre of ice. From this fact result many familiar phenomena, such as the floating of ice, the upheaving and disintegrating action of frost, and the bursting of pipes and other hollow vessels when water is frozen in them. The crystals of ice belong to the socalled hexagonal system; they are six-sided prisms, with regular faces; by agglomeration they produce stellar and fern-like forms of infinite variety and great beauty. Ice is a slow conductor of heat, and a non-conductor of electricity. It becomes electric by friction.

Steam is a colorless, transparent gas, as invisible as atmospheric air. It is lighter than air, the weight of any given volume of steam, at the ordinary temperature, being to that of the same volume of air as 0·622 to 1, a ratio deduced by calculation from the composition of steam. At 100°, the boiling-point of water, the ratio of the weights of equal volumes of steam and air is 0.455 to 1, and one volume of water furnishes about 1700 volumes of steam of 100°. When steam is heated by itself, without the presence of any liquid water, it is called superheated steam; but when there is water present, so that no excess of heat can accumulate in the steam, above the quantity needed for its formation under the pressure at which it exists, the steam is called saturated, meaning saturated with water. When steam escapes into the air, there is formed a multitude of little bubbles or vesicles, composed of a film of water filled with air, precisely similar to the vesicles seen in clouds and fogs. This steam-cloud is sometimes improperly spoken of as steam or vapor, an error against which the student should be upon his guard. Similar fogs of air-filled vesicles are formed whenever the atmosphere is cooled to a temperature so low that the aqueous vapor contained in it can no longer exist in the gaseous state.

33. Let us pass now to the analysis of water. Of what is water composed? We can determine this point by methods

similar to those which were adopted in the examination of air. There are several metals which, upon being brought into contact with water, will abstract from it one of its ingredients, in the same way that we have seen them abstract oxygen from the air. Some metals can abstract this ingredient even at the ordinary temperature. Thus the metal called sodium, on being brought into contact with water, decomposes it, and, uniting with one of its constituents, sets free another as a gas. This new gas is called Hydrogen.

Exp. 14.-Make a small cylinder of wire gauze by rolling a piece of fine gauze, about 6 c.m. square, around a thick piece of No. 6 glass tubing. Twist fine wire around the cylinder in order to preserve its form, then slip the cylinder off the glass, and close one end of it by pressure with a stout pair of pincers. Within this cylinder of wire gauze place a piece of metallic sodium as large as a small pea, and then close the upper end of the cylinder by pressure with the pincers, as before. Quickly place the wire gauze cylinder under the mouth of a small bottle of 100 or 200 c. c. capacity, which has previously been filled with water and left inverted in the water-pan.

As soon as water comes in contact with the sodium, bubbles of gas will issue from the wire gauze cage, and, rising through the water, will collect at the top of the inverted bottle. If no gas is generated during the first moment after the wire gauze has been placed in the water, move the bottle to and fro, in such manner that the gauze cylinder may be shaken about, and water forced through its interstices. As soon as the evolution of gas has ceased, close the mouth of the bottle with a small plate of glass, turn it mouth uppermost, remove the plate, and touch a lighted match to the gas. The gas will take fire with a sudden flash.

34. At a low red heat water can be decomposed by several of the metals, such as iron, tin, zinc, and magnesium. Iron is well adapted for this experiment.

Exp. 15.-Provide a piece of iron gas-pipe, about 35 c.m. long, and 1 c.m. or more in internal diameter; fill it with small, bright ironturnings, and support it upon a ring of the iron stand over one or two wire-gauze gas-lamps. By means of perforated corks, connect with the iron tube, on the one hand, a glass delivery-tube leading to the water-pan, as shown in the figure, and, upon the other, a delivery-tube coming from a thin-bottomed glass flask, half full of water, and supported upon a ring of a second iron stand. Light the lamps beneath

ELECTROLYSIS OF WATER.

23

the iron tube, and wait until its contents have become red-hot; then heat the water in the flask until it boils slowly. As the aqueous Fig. 8.

vapor passes over the hot iron-turnings it will be decomposed, one of its constituents will unite with the iron, and hydrogen will pass off through the delivery-tube and may be collected in bottles at the water-pan, so soon as the air originally contained in the tubes and flask has all been expelled.

If, at the close of this experiment, and after the tube has become cold, the iron be removed from the tube, it will be found to be covered with a black coating similar to that which forms on iron heated in the air.

35. By these experiments it has been proved that one of the components of water is a gas called hydrogen. But with the exception of the coating upon the iron of Exp. 15, we have as yet nothing to indicate what other ingredients the water may contain. Such evidence can, however, be readily obtained by resorting to another method of analysis. If a galvanic current is caused to flow through water, the force by which the constituents of the water are held together will be overcome, and the water will be resolved into the elements of which it is composed. On immersing the platinum poles of a galvanic battery in water, to which a little sulphuric acid has been added for the purpose of increasing its conducting-power, minute bubbles of gas will immediately be given off from these poles, and will be seen rising through the liquid. We have here abundant proof of the powerful action exerted by the battery upon the water. But the experiment will be much more satisfactory if it be made in a vessel so arranged that the evolved gases may be collected for examination.

24

ELECTROLYSIS OF WATER.

Fig. 9.

For this purpose the apparatus shown in Fig. 9 can be conveniently employed. The test-glass, nearly full of water which has been mixed with from to of sulphuric acid, carries two platinum wires cemented with shellac into the glass. These wires terminate above in thin plates of platinum; over each of these plates there is inverted a long, narrow test-tube full of water, acidulated in the same way as that in the test-glass. Upon connecting the wires with a galvanic battery,— two Bunsen's cells of medium size will be sufficient,—the water will be decomposed, and the resulting gases, as they are given off at the platinum plates, will rise, transparent and colorless, into the test-tubes. On comparing the bulks of the two gases, it will be found that twice as much gas has collected in the one tube as in the other. If the test-tube containing the larger volume of gas be now closed with the thumb, turned mouth uppermost, and the gas within touched with a lighted match, it will take fire and burn with the characteristic flame of hydrogen. It is, in fact, hydrogen.

If the smaller volume of gas in the other tube be examined in the same way, it will not inflame, although it gives intense brilliancy to the combustion of the match. If a splinter of wood, retaining but a single ignited spark, be immersed in the gas, it instantly exhibits a vivid incandescence, and in a moment bursts into flame. This gas is oxygen. It is thus proved that out of water may be unloosed two volumes of hydrogen and one volume of oxygen.

If now the platinum plates be pressed so near together that a single large test-tube, full of acidulated water, can be placed over both, the gas obtained by passing the galvanic current will exhibit properties differing from those of either hydrogen or oxygen. It is in fact a mechanical mixture of these gases in the proportions in which they would unite chemically to form water. The mixture is precisely

similar to that which would have been obtained if the two volumes of hydrogen and one volume of oxygen, previously collected in two separate tubes, had been mixed in one. On touching a lighted match to the mixed gas it instantly explodes with great violence, the hydrogen burning suddenly, so that for a moment a flash of flame fills the whole interior of the tube. Incited by the burning match, the hydrogen and oxygen have combined chemically to form water, a portion of which is deposited as dew upon the inner walls of the tube.

At the temperature of the air, and under ordinary circum