-------+----------------------+------------------------- | CARBONIC ACID. | | PARTS PER MILLION | Year. +----------+-----------+ Nature of Treatment. |Raw Water.|Chlorinated| | | Water. | -------+----------+-----------+------------------------- 1915 | 1.44 | 1.41 | Bleach 1916 | 0.92 | 0.85 | Bleach 1917 | 0.84 | 0.81 | Bleach first four months | | | Chloramine during last | | | eight months -------+----------+-----------+------------------------- These figures shown that the hypochlorite treatment produced a small but definite decrease in the carbonic acid content and should, _cæteris paribus_, tend to reduce and not increase corrosion. If the corrosion of pipes is considered according to the electrolytic theory, a slight increase, due to an increased electrical conductivity, might be anticipated. The effect of the addition of hypochlorite upon the electrical conductivity of distilled water and the Ottawa River water is shown in Diagram VI. [Illustration: DIAGRAM VI Effect of Calcium Hypochlorite on Electrical Conductivity] With the concentrations of hypochlorite ordinarily used in water treatment it is inconceivable that the slight increase in the electrical conductivity has any practical significance at low temperatures. The conductivity increases rapidly, however, with increase of temperature and any increment due to chlorination might produce a slight appreciable effect at temperatures approaching the boiling-point of water. Liquid chlorine does not increase the conductivity to the same extent as an equivalent quantity of hypochlorite but it increases the carbonic acid content in proportion to the dosage used. The author investigated the action of hypochlorite on galvanised pipes in 1914 and was unable to detect any definite corrosion with normal concentrations of chlorine. The experiments were made with 2-inch pipes and an examination of the first consignment received showed that, although the galvanising on the outside was perfect, the inner coat was very inferior: in some parts there was an excess of zinc that broke away on scraping whilst in others the iron pipe was bare. A committee of the Pittsburg Board of Trade, appointed to investigate complaints as to pipe corrosion, reported in 1917 that they were largely due to inferior qualities of pipes and not to the method of water purification employed (slow sand filtration and chlorination).