Few accurate researches have been made on the effect of the impurities of water on tanning,[65] and though, from what has already been said, it will be seen that they are not without effect it is probable that in many cases the water is blamed for troubles which are simply the result of mismanagement, and credited with virtues which are really due to careful and skilful manufacture. [65] See Nihoul, ‘Influence de l’eau sur l’extraction des matières tannantes,’ Bulletin de la Bourse aux Cuirs de Liège, Sept. 1901. The hardness of water, and the dissolved carbonic acid which it contains, are, together with its temperature, the principal factors which determine whether a hide will plump or fall in it. Almost the only accurate investigation of this point has been made by W. Eitner.[66] He placed pieces of hide, unhaired by sweating, and quite flat and fallen, in water for four days at a temperature of 46° F. (8° C.), with the following results:-- 1. In distilled water Scarcely at all plumped. 2. „ water saturated with CO₂ Well plumped. 3. „ „ with lime bicarbonate, 20° German} Tolerably plump. scale of hardness } 4. „ „ „ magnesia bicarbonate, 20° do. „ „ 5. „ „ „ lime sulphate 20° do. Well plumped. 6. „ „ „ magnesia sulphate, 20° do Best plumped. 7. „ „ „ magnesium chloride, 20° do. Not at all plumped. 8. „ „ „ common salt, 20° do. „ „ (1 German degree of hardness corresponds to 1 of CaO in 100,000.) [66] Gerber, iii. (1877) p. 183.