As already pointed out the liquors contain a certain amount of soda, as sodium sulphide and other sulphur compounds. The presence of the former, if in large quantities, is objectionable, as it is liable to discolour fibres boiled in liquors containing it. It is therefore best to remove it. This can be conveniently done by blowing air into the liquors during the process of causticising: this has the effect of oxidising it to sulphate of soda, in which form it is harmless. The air can of course be blown into the liquor by means of a pump; the most economical way is to connect with the steam pipe an injector constructed on the principle of the injectors used for feeding boilers and for other purposes. By this means a strong current of air is drawn in, and {192} being forced with the stream to the bottom of the liquor, passes through it in a number of fine streams. If the amount of sulphide present be very high it may be necessary to prolong the oxidising operation beyond the time necessary for complete causticising. In many paper-mills the causticising is conducted in circular vessels furnished with mechanical agitators. These are more expensive than the simple form described above, and they possess no special advantages. The use of causticisers in which neither mechanical agitation nor agitation by means of air is provided for is exceedingly wasteful of labour, time, steam, and soda. The lime-mud settles at the bottom as a hard mass, very difficult to manipulate. {193} CHAPTER XIII. PAPER TESTING. There are two points of view from which a paper may be tested: first, of physical or mechanical properties; secondly, of material composition. We shall consider the subject according to this division. (I.) Quantitative measurements of such properties as resistances to breaking and tearing strains are seldom made by English paper-makers. In Germany, on the other hand, the matter has been very thoroughly investigated in connection with the work of the Königl. Techn. Versuchsanstalt, Berlin, and through the agency and influence of Prof. Sell, and C. Hofmann, a department has been organised exclusively for the work of paper testing. The results of the tests are becoming widely recognised by practical men and the trade in that country, as affording a true index of the quality of a paper. It is therefore of importance to give an outline of the methods employed.