Steeping. The procedure to be observed is as follows. The rapidity with which the salts may be removed varies directly with the quantity of water used in steeping. The treatment of objects of small size presents no difficulties; any vessel of glass, porcelain, or earthenware will serve the purpose. Towards the end of the treatment distilled water should be used, or in default of this, clean rain water should be used in preference to that from a well. For larger objects (as, for example, the large limestone blocks of the Meten Chamber mentioned above, some of which were 1 metre in length and 1⁄2 metre or more in breadth and thickness), it is convenient to use wooden tubs fitted with a tap in front to draw off the water, and so tilted by means of stones placed underneath that the tap may be at the lowest point. The objects should not touch the bottom of the vessel. Smaller pieces may be suspended or may be made to rest on glass rings or supports of glass rods; while large objects should be laid on blocks of wood so placed as to allow the tub to be cleaned when necessary without removal of the blocks, the weight of which would otherwise entail much labour. The blocks should be as near to the surface of the water as possible, leaving a considerable depth of water beneath, for the heavier salt-laden water sinks to the bottom, thus bringing into contact with the limestone water with a smaller salt-content. The length of the steeping must depend upon the size and porosity of the limestone.
Under certain circumstances phenomena make their appearance which must not be neglected. Thus if the treatment extends over a considerable length of time, the wooden tubs should be provided with lids to prevent the access of light. This was found indispensable in the treatment of the blocks from the Meten Chamber when Berlin tap-water[83] was used, for when the tubs were open a large quantity of brown hydrated ferric oxide appeared on the limestone, the roughness of which rendered its removal an impossibility even with brushes. This oxide is produced by various forms of algae and bacteria which developed in such numbers that the sides of the tubs were frequently covered with a layer of slime, which under the microscope appeared as a confused web of transparent threads[84]. This was brushed off with soft brushes at least once a fortnight, for the slimy covering impeded the access of the water to the limestone. That in this case the ferric oxide was the result of the action of light was proved by the fact that only those blocks which were placed near the windows were discoloured, and that the discolouration was proportionate to the amount of light which fell upon them[85]. Again, after the treatment in the covered tubs some of the blocks became so black that they resembled blocks of coal rather than limestone. After exposure to light for a day or two, especially when the water had been drawn off, the discolouration disappeared without leaving any traces. The colour was doubtless due to a minute quantity of iron in the form of sulphide which, after oxidation in the air and light, became invisible upon the light yellow limestone. Under these circumstances the presence of sulphuretted hydrogen in the water, possibly produced by bacterial action upon the sulphates, was attested by the characteristic smell.
The enormous number of bacteria which develop in the water constitute a great hindrance to the process of steeping, and as to boil such a quantity of water as is required for these large objects is out of the question, frequent changes of water and frequent cleaning of the stone, wooden blocks, and tubs are the only remedies.
Examination of the Progress of the Steeping. The water should be changed at first daily, then by degrees every two, three, or four days, later on weekly only, until finally once a fortnight is sufficient. To ascertain the progress and completion of the elimination the quantity of chlorine in the wash-water may be determined by a simple method of titration[ [86].
The following short explanation may be of use, and the method is easily learnt. If a solution of silver nitrate is poured into a solution of common salt (sodium chloride), a white curdy precipitate is produced, a process which the following equation will explain:
NaCl + AgNO3 = AgCl + NaNO3.
The white precipitate is the silver chloride, whilst the sodium nitrate which is produced at the same time remains in solution and is therefore not visible. As always definite proportions of the two substances, silver nitrate and sodium chloride, react upon one another, by the use of a solution containing a known amount of silver nitrate we can determine the amount of salt, and hence of chlorine present. By cautiously inclining a burette (Fig. [13]) divided into tenths of cubic centimetres[87], the silver solution should be dropped into a beaker containing a definite volume of the solution to be examined for chlorine; the level of the silver solution in the burette should be read off before and after pouring out, and the number of cubic centimetres of the silver solution required to precipitate the chlorine will thus be known.
Fig. 13. Gay-Lussac’s Burette. 1⁄6 nat. size.