[84] This was not the well-known Crenothrix only. Cp. “Polytechn. Centralblatt,” 1891-92, p. 195. (See footnote, [p. 10.])
[85] It has been found that the formation of this layer of slime may be avoided by the use of tubs which are lined with sheet zinc. The addition of 10-20 cubic centimetres of formalin [40% solution of formaldehyde] to every hundred gallons of water also prevents or restrains the formation of slime. It is not necessary to add formalin each time the water is changed.
[86] Since chlorine compounds (especially common salt) form the predominating substance in the soluble salts contained in limestones their removal may be considered a proof that other salts (e.g. sulphates) have also been removed. Hence it is sufficient to prove the disappearance of chlorine. In the rare cases in which sulphates only are present, a test similar to that mentioned on [p. 77], applied to clay objects, should be used. If the water used for soaking salt-containing limestone, earthenware, etc., gives no precipitate, or only turbidity with the silver solution, the determination of chlorine by titration is not applicable.
[87] Though some other kinds of burette may be easier to use, that here recommended (that of Gay-Lussac) is the most convenient for reasons into which we need not enter. The following precautions should be observed: where the burette is not closed by a cork, let a few drops out first to wash away crystals of silver nitrate which may have formed at the mouth. The silver solution should be kept in well stoppered bottles. When filling the burette a glass funnel should be used, so that the cork used for closing the burette is not wetted with the silver solution. Before reading off wait until the level of the fluid is constant, in order that any solution on the sides of the glass tube may have time to run down.
[89] I have found that the amount of chlorine is smaller in winter than in summer. In the summer of 1894, 100 c.c. tap-water from the Stralau Waterworks often required 0·8 c.c. silver solution: but at that time stronger disinfectants were used on account of the cholera, and this may have caused the increase of chlorine; for since then, and even at the present time (winter 1898), 100 c.c. tap-water requires 0·5 to 0·6 c.c. silver solution.
[90] It may be here observed that objects of limestone or of earthenware may be numbered or marked at the back in black iron ink, which does not disappear even after prolonged steeping in water.
[91] Lepsius, “Denkmäler aus Aegypten und Aethiopien.”
[92] It is scarcely necessary to add that any other form of air-pump may be used.
[93] A powerful pressure of water [in combination with a well-acting pump] may cause the fluid to evaporate with such rapidity as to produce bubbles, but these bubbles are easily distinguished by their size from the minute bubbles of air. To avoid this ebullition, the air should not be pumped out too rapidly.