other words, the volume of ice at the zero of temperature would not be the minimum volume of the water molecule, though we have every reason to believe it would be so in the case of the majority of known substances. Another substance of special interest is solid carbonic acid. This body has a density of 1.53 at -78° C. and 1.633 at -185° C., thus giving a mean coefficient of expansion between these temperatures of 0.00057. This value is only about 1/6 of the coefficient of expansion of the liquid carbonic acid gas just above its melting point, but it is still much greater at the low temperature than that of highly expansive solids like sulphur, which at 40° C. has a value of 0.00019. The following table gives the densities at the temperature of boiling liquid air (-185° C.) and at ordinary temperatures (17° C.), together with the mean coefficient of expansion between those temperatures, in the case of a number of hydrated salts and other substances: TABLE I. +---------------------------+---------+--------+------------+ | | | | Mean | | | | | coefficient| | | Density | Density|of expansion| | | at -185°| at +17°| between | | | C. | C. |-185° C. and| | | | | +17° C. | +---------------------------+---------+--------+------------+ | Aluminium sulphate (18)* | 1.7194 | 1.6913 | 0.0000811 | | Sodium biborate (10) | 1.7284 | 1.6937 | 0.0001000 | | Calcium chloride (6) | 1.7187 | 1.6775 | 0.0001191 | | Magnesium chloride (6) | 1.6039 | 1.5693 | 0.0001072 | | Potash alum (24) | 1.6414 | 1.6144 | 0.0000813 | | Chrome alum (24) | 1.7842 | 1.7669 | 0.0000478 | | Sodium carbonate (10) | 1.4926 | 1.4460 | 0.0001563 | | Sodium phosphate (12) | 1.5446 | 1.5200 | 0.0000787 | | Sodium thiosulphate (5) | 1.7635 | 1.7290 | 0.0000969 | | Potassium ferrocyanide (3)| 1.8988 | 1.8533 | 0.0001195 | | Potassium ferricyanide | 1.8944 | 1.8109 | 0.0002244 | | Sodium nitro-prusside (4) | 1.7196 | 1.6803 | 0.0001138 | | Ammonium chloride | 1.5757 | 1.5188 | 0.0001820 | | Oxalic acid (2) | 1.7024 | 1.6145 | 0.0002643 | | Methyl oxalate | 1.5278 | 1.4260 | 0.0003482 | | Paraffin | 0.9770 | 0.9103 | 0.0003567 | | Naphthalene | 1.2355 | 1.1589 | 0.0003200 | | Chloral hydrate | 1.9744 | 1.9151 | 0.0001482 | | Urea | 1.3617 | 1.3190 | 0.0001579 | | Iodoform | 4.4459 | 4.1955 | 0.0002930 | | Iodine | 4.8943 | 4.6631 | 0.0002510 | | Sulphur | 2.0989 | 2.0522 | 0.0001152 |