Mass Production of Watches

A water with much temporary hardness can be softened by adding just sufficient milk of lime to combine with the excess of carbon dioxide, whereby all the chalk is precipitated. Many water softeners are marketed under various names, e.g., "Sofnol," "Per mutite"; the latter is a natural or artificial zeolite (q.v.) which exchanges sodium for calcium when a hard water is passed over it, and which is "revived" by contact with brine. The purest water ordinarily obtainable is procured by distillation (q.v.) which leaves all solid impurities in the still.

Pure Water.

Absolutely pure water is probably unknown "Chemically pure" water of the laboratory is extremely difficult to prepare. Ordinarily pure water is used as a standard for many weights and measures. Thus, a gallon is defined as the volume at 62° F of iolb. of water when weighed in air at 3oin. barometric pressure and at 62° F. Similarly, the original kilogram was sup posed to be the mass of 1 cu.dm. of water at 4° C. (It is now known that the kilogram is not strictly what it was supposed to be; consequently, it has to be redefined as the mass of a particular standard—the "kilogramme des archives." This means that the litre, which is based on the kilogram, is not strictly ',coo cu.cm. and the tendency is to use the millilitre instead of the cubic cen timetre for very accurate work : 1 ml. = 1.000027 cu.cm.) The fixed points on thermometric scales are based on the melting point of ice (o° C; 32° F) and the boiling point of water (100° C at 76omm. pressure; 212° F at 3oin. pressure). Specific heats are usually referred to that of water as unity, because the unit of heat is based on the heat capacity of water. The specific heat of water (which is about 1% greater at o° and at Ioo° than at 25°) is greater than that of almost all other liquids (liquid ammonia is the chief exception) ; it is owing to this property of water that insular climates are subject to less extreme changes of tempera ture than continental climates.

Another remarkable property of water is its increasing density, on being cooled, to a maximum at 4° C, after which further cool ing causes it to expand, and on freezing it expands rapidly. The following are the figures of this change in density :— Water at io° C=0.99973 Water at oo° C --= 0.9585 Ice at o°Water at 0° C=0.99987 Water at 4° C=I•00000 (by definition of kilogram).

The significance of this is dealt with in the article ICE.

On conversion into steam at 100°C, one volume of water ex pands about 1,700 times under ordinary pressure and absorbs 538 calories per gram as "latent heat of vaporisation." Its critical temperature is 370° C—whatever the pressure, it cannot be lique fied above this temperature. The peculiar ability of water to initiate or to facilitate (i.e., to "catalyze") chemical reactions is

described in the article DRYNESS, CHEMICAL; it is ascribed by H. E. Armstrong to its dissolution of traces of the reactants or of "impurities" to form conducting solutions.

Constitution.

In former times, water was regarded as an "element," and even when this term acquired its present usage the idea was not abandoned, although water had been decomposed in various ways. It was not until Cavendish prepared pure hydro gen and burnt it in air to form water that it came to be recognized as a compound. He further showed it to be formed from 2.014 vols. of hydrogen and i vol. of oxygen, and it was only many years later that the researches of E. W. Morley, A. Scott, Leduc, Lord Rayleigh and others showed that this slight deviation from the theoretical ratio (2:I) is due to the two gases showing slight deviations from Boyle's law at ordinary pressures (see STOICHIO METRY) . The gravimetric composition is 2.0154 parts of hydrogen to 16.000 parts of oxygen.

It seems probable that liquid water is a mixture of several types of molecules (ILO), and especially (11,0), predominating. Many of its physical properties point to such a conclusion (see ASSOCIA TION) thus, the latent heats of fusion and vaporisation (vide supra) are much higher than those of most other liquids, and this is attributed to the heat absorbed in breaking down complex into simpler molecules. The ice molecule—possibly un doubtedly bulky (witness the low density of ice, q.v.), and the occurrence of a point of maximum density of water is probably due to the normal expansion of the liquid being outweighed by the contraction due to these molecules, which have persisted in the liquid, breaking down to less bulky (11,0), molecules.

The foregoing ideas have been variously expressed by different chemists. Thus, I. Traube postulated the existence of "gasogenic" (simple molecules, and "liquidogenic" (complex) molecules; and A. W. C. Menzies (1921) confirmed that steam at 'co° C contains only molecules. W. Sutherland, from a considera tion of all the physical properties of ice, water and steam, as signed values for these properties to each of the molecular species (HD), and and calculated that water at 0°C con tained 37.5% of "trihydrol" and 62.5% of "dihydrol." H. E. Arm strong believes that, among other molecules in water, the molecules are of at least two types dihydrone (inactive) and hydronol (active), respectively. Finally, A. Hantzsch believes water to be a basic anhydride of the oxonium hydroxide ELO.OH in conformity with oxonium salts such as and the so-called "hydrogen ion," which is possibly (see ACIDS). (A. D. M.)