Lead (and copper) may be detected by passing sulphuretted hydrogen through the water acidified with HCl, or by adding a drop of fresh ammonium or sodium sulphide to the slightly acidified water, when a brownish coloration clearly visible in a deep beaker set on a sheet of white paper will be produced. Iron also gives a black with sulphides in alkaline solution. Copper may be distinguished from lead by the blue given with ammonia, and by a reddish-brown precipitate with potassium ferrocyanide. For accurate quantitative estimation of these impurities, the regular works on the subject, such as Thorpe's 'Quantitative Analysis,' Sutton's 'Volumetric Analysis,' or Fresenius' 'Quantitative Analysis,' must be consulted. EXAMINATION OF COMMERCIAL ACIDS. _Sulphuric acid_ 10 _grm._ may be made up to 100 _c.c._ and well mixed, and of this 10 _c.c._ may be tested with normal sodic carbonate in presence of methyl orange. Each 1 _c.c._ of soda solution used corresponds to 0·049 _grm._ or 4·9 per cent. of H_{2}SO_{4}. For most purposes, the strength may be ascertained from the specific gravity, as measured by a hydrometer or weighed in a specific gravity bottle. The following table gives the strength at 59° F. (15° C.):-- ─────────┬───────────────┬──────────── Specific │ Degrees │ Per cent. Gravity. │ Twaddell.[H] │ H_{2}SO_{4} ─────────┼───────────────┼──────────── │ ° │ 1·8426 │ 168·5 │ 100 1·8376 │ 167·5 │ 95 1·822 │ 164 │ 90 1·786 │ 157 │ 85 1·734 │ 147 │ 80 1·675 │ 135 │ 75 1·615 │ 123 │ 70 1·557 │ 111 │ 65 1·501 │ 100 │ 60 1·448 │ 90 │ 55 1·398 │ 80 │ 50 1·351 │ 70 │ 45 1·306 │ 61 │ 40 1·264 │ 53 │ 35 1·223 │ 45 │ 30 1·182 │ 36 │ 25 1·144 │ 29 │ 20 1·106 │ 21 │ 15 1·068 │ 14 │ 10 1·032 │ 6 │ 5 ─────────┴───────────────┴──────────── [Footnote H: Degrees of Twaddell's hydrometer may be reduced to specific gravity by multiplying by ·005 and adding 1·, thus 10° Tw. = 1·050 sp. gr.] The impurities of sulphuric acid most common and injurious for tanning purposes are iron and nitrous acid. Iron is detected on neutralising with soda or ammonia, when it falls as a yellowish precipitate, which may be recognised by the ordinary tests (p. 100). Nitric and nitrous acids are detected by pouring a strong solution of ferrous sulphate cautiously on to the top of the strong cold acid, when a dark ring is formed at the junction of the two liquids. _Hydrochloric acid_ may be tested with soda solution like sulphuric. 1 _c.c._ of normal soda = 0·0365 _grm._ or 3·65 per cent. HCl. It may also be calculated from specific gravity.