The English degree is a survival of the time when the atomic weight of sodium was believed to be twenty-four instead of twenty-three, and, since the error on 76 per cent. Na_{2}O due to this amounts to about 1 per cent., may be obtained by adding this figure to the sodium oxide really present. _Caustic soda_ (sodium hydrate) comes into commerce in a liquid form as 90 deg. Tw. (and even as high as 106 deg. Tw.), and other degrees of dilution, and also in a solid form in various grades as 60 deg., 70 deg., 76-77 deg., 77-78 deg. These degrees represent the percentage of sodium oxide (Na_{2}O) present plus the 1 per cent. The highest grade, containing as it does more available caustic soda and less impurities, is much more advantageous in use. _Carbonate of soda_ or _soda ash_, 58 deg., also termed "light ash," and "refined alkali". This is a commercially pure sodium carbonate containing about 0.5 per cent. salt (NaCl). The 58 deg. represents the English degrees and corresponds to 99 per cent. sodium carbonate (Na_{2}CO_{3}). _Soda ash_, 48 deg., sometimes called "caustic soda ash," often contains besides carbonate of soda, 4 per cent. caustic soda (sodium hydrate), and 10 per cent. salt (sodium chloride), together with water and impurities. The 48 degrees refers to the amount of alkali present in terms of sodium oxide (Na_{2}O), but expressed as English degrees. _Caustic potash_ (potassium hydrate) is offered as a liquid of 50-52 deg. B. (98-103 deg. Tw.) strength, and also in solid form as 75-80 deg. and 88-92 deg. The degrees in the latter case refer to the percentage of potassium hydrate (KHO) actually present. _Carbonate of Potash._--The standard for refined carbonate of potash is 90-92 per cent. of actual potassium carbonate (K_{2}CO_{3}) present, although it can be obtained testing 95-98 per cent. OTHER MATERIALS. _Water._--Water intended for use in soap-making should be as soft as possible. If the water supply is hard, it should be treated chemically; the softening agents may be lime and soda ash together, soda ash alone, or caustic soda. There are many excellent plants in vogue for water softening, which are based on similar principles and merely vary in mechanical arrangement. The advantages accruing from the softening of hard water intended for steam-raising are sufficiently established and need not be detailed here. _Salt_ (sodium chloride or common salt, NaCl) is a very important material to the soap-maker, and is obtainable in a very pure state. Brine, or a saturated solution of salt, is very convenient in soap-making, and, if the salt used is pure, will contain 26.4 per cent. sodium chloride and have a density of 41.6 deg. Tw. (24.8 deg. B.). The presence of sulphates alters the density, and of course the sodium chloride content. Salt produced during the recovery of glycerine from the spent lyes often contains sulphates, and the density of the brine made from this salt ranges higher than 42 deg. Tw. (25 deg. B.).