[58] Offer (1880) concludes, from his researches on cryohydrates, that they are simple mixtures of ice and salts, having a constant melting point, just as there are alloys having a constant point of fusion, and solutions of liquids with a constant boiling point (_see_ note 60). This does not, however, explain in what form a salt is contained, for instance, in the cryohydrate NaCl + 10H_{2}O. At temperatures above -10° common salt separates out in anhydrous crystals, and at temperatures near -10°, in combination with water of crystallisation, NaCl + 2H_{2}O, and, therefore, it is very improbable that at still lower temperatures it would separate without water. If the possibility of the solidified cryohydrate containing NaCl + 2H_{2}O and ice be admitted, then it is not clear why one of these substances does not melt before the other. If alcohol does not extract water from the solid mass, leaving the salt behind, this does not prove the presence of ice, because alcohol also takes up water from the crystals of many hydrated substances (for instance, from NaCl + 2H_{2}O) at about their melting-points. Besides which, a simple observation on the cryohydrate, NaCl + 10H_{2}O, shows that with the most careful cooling it does not on the addition of ice deposit ice, which would occur if ice were formed on solidification intermixed with the salt. I may add with regard to cryohydrates that many of the solutions of acids solidify completely on prolonged cooling (for example, H_{2}SO_{4},H_{2}O), and then form perfectly definite compounds. For the solutions of sulphuric acid (_see_ Chapter XX.) Pickering obtained, for instance, a hydrate, H_{2}SO_{4},4H_{2}O at -25°. Hydrochloric, nitric, and other acids also give similar crystalline hydrates, melting at low temperatures and presenting many similarities with the cryohydrates. [59] _See_ note 24.