[8] One of Lavoisier's first memoirs (1770) referred to this question. He investigated the formation of the earthy residue in the distillation of water in order to prove whether it was possible, as was affirmed, to convert water into earth, and he found that the residue was produced by the action of water on the sides of the vessel containing it, and not from the water itself. He proved this to be the case by direct weighing.
[9] Taking the generally-accepted specific gravity of water at its greatest density—i.e. at 4° as one—it has been shown by experiment that the specific gravity of water at different temperatures is as follows:
| At | 0° | 0·99987 | At | 30° | 0·99574 | |
| „ | +10° | 0·99974 | „ | 40° | 0·99233 | |
| „ | 15° | 0·99915 | „ | 50° | 0·98817 | |
| „ | 20° | 0·99827 | „ | 100° | 0·95859 |
A comparison of all the data at present known shows that the variation of the specific gravity St with the temperature t (determined by the mercurial thermometer) maybe expressed (Mendeléeff 1891) by the formula
St = 1 - (t - 4)2 / (94·1 + t) (703·51 - t) 1·9
| t° C. according to the mercurial thermometer | Sp. gr. St (at 4° = 1,000,000) | Variation of sp. gr. with a rise of | Volume taking vol. at 4° = 1 | |
| Temp. per 1° C. or ds/dt | Pressure per 1 atmosphere or ds/dp | |||
| -10 | 998,281 | +264 | +54 | 1,001,722 |
| 0 | 999,873 | +65 | +50 | 1,000,127 |
| 10 | 999,738 | -85 | +47 | 1,000,262 |
| 20 | 998,272 | -203 | +45 | 1,001,731 |
| 30 | 995,743 | -299 | +43 | 1,004,276 |
| 50 | 988,174 | -450 | +40 | 1,011,967 |
| 70 | 977,948 | -569 | +39 | 1,022,549 |
| 90 | 965,537 | -670 | +41 | 1,035,692 |
| 100 | 958,595 | -718 | +42 | 1,043,194 |
| 120 | 943,814 | -819 | +43 | 1,060,093 |
| 160 | 907,263 | -995 | +55 | 1,102,216 |
| 200 | 863,473 | -1,200 | +73 | 1,158,114 |
If the temperature be determined by the hydrogen thermometer, whose indications between 0° and 100° are slightly lower than the mercurial (for example, about 0·1° C. at 20°), then a slightly smaller sp. gr. will be obtained for a given t. Thus Chappuis (1892) obtained 0·998233 for 20°. Water at 4° is taken as the basis for reducing measures of length to measures of weight and volume. The metric, decimal, system of measures of weights and volumes is generally employed in science. The starting point of this system is the metre (39·37 inches) divided into decimetres (= 0·1 metre), centimetres (= 0·01 metre), millimetres (= 0·001 metre), and micrometres (= one millionth of a metre). A cubic decimetre is called a litre, and is used for the measurement of volumes. The weight of a litre of water at 4° in a vacuum is called a kilogram. One thousandth part of a kilogram of water weighs one gram. It is divided into decigrams, centigrams, and milligrams (= 0·001 gram). An English pound equals 453·59 grams. The great advantage of this system is that it is a decimal one, and that it is universally adopted in science and in most international relations. All the measures cited in this work are metrical. The units most often used in science are:—Of length, the centimetre; of weight, the gram; of time, the second; of temperature, the degree Celsius or Centigrade. According to the most trustworthy determinations (Kupfer in Russia 1841, and Chaney in England 1892), the weight of a c. dcm. of water at 4° in vacuo is about 999·9 grms. For ordinary purposes the weight of a c. dcg. may be taken as equal to a kg. Hence the litre (determined by the weight of water it holds) is slightly greater than a cubic decimetre.
[10] As solid substances appear in independent, regular, crystalline forms which are dependent, judging from their cleavage or lamination (in virtue of which mica breaks, up into laminae, and Iceland spar, &c., into pieces bounded by faces inclined to each other at angles which are definite for each substance), on an inequality of attraction (cohesion, hardness) in different directions which intersect at definite angles the determination of crystalline form therefore affords one of the most important characteristics for identifying definite chemical compounds. The elements of crystallography which comprise a special science should therefore he familiar to all who desire to work in scientific chemistry. In this work we shall only have occasion to speak of a few crystalline forms, some of which are shown in figs. [6] to [12].
[11] Of all known liquids, water exhibits the greatest cohesion of particles. Indeed, it ascends to a greater height in capillary tubes than other liquids; for instance, two and a half times as high as alcohol, nearly three times as high as ether, and to a much greater height than oil of vitriol, &c. In a tube one mm. in diameter, water at 0° ascends 15·3 mm., measuring from the height of the liquid to two-thirds of the height of the meniscus, and at 100° it rises 12·5 mm. The cohesion varies very uniformly with the temperature; thus at 50° the height of the capillary column equals 13·9 mm.—that is, the mean between the columns at 0° and 100°. This uniformity is not destroyed even at temperatures near the freezing point, and hence it may be assumed that at high temperatures cohesion will vary as uniformly as at ordinary temperatures; that is, the difference between the columns at 0° and 100° being 2·8 mm., the height of the column at 500° should be 15·2 - (5 × 2·8) = 1·2 mm.; or, in other words, at these high temperatures the cohesion between the particles of water would he almost nil. Only certain solutions (sal ammoniac and lithium chloride), and these only with a great excess of water, rise higher than pure water in capillary tubes. The great cohesion of water doubtless determines many of both its physical and chemical properties.