Water, purified as above described, has constant _physical_ and _chemical properties_. For instance, it is of such water only that one cubic centimetre weighs one gram at 4° C.--_i.e._ it is only such pure water whose specific gravity equals 1 at 4° C.[9] Water in a solid state forms crystals of the hexagonal system[10] which are seen in snow, which generally consists of star-like clusters of several crystals, and also in the half-melted scattered ice floating on rivers in spring time. At this time of the year the ice splits up into spars or prisms, bounded by angles proper to substances crystallising in the hexagonal system. [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 S_{t} with the temperature _t_ (determined by the mercurial thermometer) maybe expressed (Mendeléeff 1891) by the formula S_{t} = 1 - (_t_-4)^{2}/(94·1 + _t_) (703·51-_t_) 1·9 +-----------+-------------+---------------------------+-----------+ | t° C. | | Variation of sp. gr. with | | | according |Sp. gr. S_{t}| a rise of | Volume | | to the |(at 4° = +--------------+------------+taking vol.| | mercurial | 1,000,000) |Temp. per 1°C.|Pressure per| at 4° = 1 | |thermometer| | or ds/dt |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 | -810 | +43 | 1,060,093 | | 160 | 907,263 | -995 | +55 | 1,102,216 | | 200 | 863,473 | -1,200 | +73 | 1,158,114 | +-----------+-------------+--------------+------------+-----------+