This section is from the book "British Homoeopathic Pharmacopoeia", by The British Homoeopathic Society. Also available from Amazon: British Homoeopathic Pharmacopoeia.
Nothing but the purest distilled water must ever be used in the preparation of any of the medicines. The ordinary distilled water sold by wholesale druggists is quite inadmissible, from the fact of its being frequently distilled in stills that are used for distilling aromatic waters, and hence it cannot be sufficiently pure for our purpose.
All the water used by homoeopathic chemists for the purpose of attenuations or for reducing the strength of rectified spirit must be distilled in an apparatus made entirely of glass or porcelain.* The apparatus should never be much more than half filled with water, and the distillation should be carried on at a gentle heat, so as to guard against any of the water boiling over. Whatever quantity is distilled, the first 20th part should be rejected, and only 16 1/2 parts should be carried over. For example, in distilling 10 pints, the first 10 fluid ounces would be thrown away, and the next 8 pints would be preserved, after which the process would be stopped.
Tests. - It possesses neither colour, taste, nor smell. Evaporated in a clean glass capsule, it leaves no visible residue. It is not affected by Sulphuretted Hydrogen, Oxalate of Ammonia, Nitrate of Silver, Chloride of Barium, or Solution of Lime.