=Acetic Acid=, C₂H₄O₂ = 58.—The very dilute form of this substance is known commonly as vinegar, the stronger as pyroligneous acid, and the purest as glacial acetic acid; the latter is, however, scarcely used. Formerly in colour making ordinary vinegar was used, but now pyroligneous acid is almost exclusively employed. This is distinguished by its strong empyreumatic smell, which, however, is without importance in colour making. The strength of a solution of acetic acid cannot be found by a simple estimation of specific gravity, since the density does not increase with the percentage of acetic acid. If an accurate estimation of the strength of acetic acid is required, it must be obtained by neutralising the acid with an alkali by a process of volumetric analysis. For practical purposes, where it is generally known whether a very strong or a more dilute acetic acid is under consideration, the following table, showing the connection between specific gravity and percentage strength, is sufficient. +--------+---------+--------+---------+ |Specific| Acetic |Specific| Acetic | |Gravity.| Acid |Gravity.| Acid | | |per cent.| |per cent.| +--------+---------+--------+---------+ | 1·0635 | 100 | 1·058 | 48 | | 1·0670 | 98 | 1·055 | 46 | | 1·0690 | 96 | 1·054 | 44 | | 1·0706 | 94 | 1·052 | 42 | | 1·0716 | 92 | 1·051 | 40 | | 1·0730 | 90 | 1·049 | 38 | | 1·0730 | 88 | 1·047 | 36 | | 1·0730 | 86 | 1·045 | 34 | | 1·0730 | 84 | 1·042 | 32 | | 1·0730 | 82 | 1·040 | 30 | | 1·0735 | 80 | 1·038 | 28 | | 1·0732 | 78 | 1·035 | 26 | | 1·0730 | 76 | 1·033 | 24 | | 1·0720 | 74 | 1·031 | 22 | | 1·0710 | 72 | 1·027 | 20 | | 1·0700 | 70 | 1·025 | 18 | | 1·0700 | 68 | 1·023 | 16 | | 1·0690 | 66 | 1·020 | 14 | | 1·0680 | 64 | 1·017 | 12 | | 1·0670 | 62 | 1·015 | 10 | | 1·0670 | 60 | 1·012 | 8 | | 1·0660 | 58 | 1·008 | 6 | | 1·0640 | 56 | 1·005 | 4 | | 1·0630 | 54 | 1·002 | 2 | | 1·0620 | 52 | 1·001 | 1 | | 1·0600 | 50 | —— | —— | +--------+---------+--------+---------+ =Oxalic Acid=, C₂H₂O₄.2H₂O = 126, has but a limited use in colour making. It comes into commerce in the form of more or less pure white crystals which readily dissolve in water, and are almost pure oxalic acid, containing only small quantities of oxalate of lime, the presence of which is without importance for the purposes to which the acid is put in colour making. Frequently, instead of oxalic acid, the acid potassium oxalate (salt of sorrel) is used.