2 (CaS_{2}O_{3} + 2 CaS_{5}) + 9 (Ca(OCl)_{2}) = 6 CaSO_{4} + Calcium Calcium Calcium Calcium thiosulphate. pentasulphide. hypochlorite. sulphate. 18 S + 9 CaCl_{2} Sulphur. Calcium chloride. {130} From this it is seen that a large quantity of free sulphur is formed, which is precipitated as a fine yellowish powder in the fibre. This not only affects the colour of the pulp, but is objectionable on account of its liability, in contact with the hot cylinders of the paper machine, or by the action of moisture over a long period of time to become oxidised to sulphuric acid, which undoubtedly, even if present only in very small quantities, has an injurious effect upon the finished paper. (See hydracellulose.) This property of sulphur and its action upon cellulose may be shown by the following experiment. Take a piece of water-leaf paper, rub into it a mixture of flour of sulphur and water. If the paper thus treated be rapidly dried, it will be found to be weakened or even slightly charred in those portions which have been in contact with the sulphur. (See also Chem. Soc. Journ., p. 249, 1879.) Whichever variety of “antichlor” is used an excess should be carefully avoided, as all act more or less upon the size and colouring matter added to the pulp subsequently. The proper method is to run in small quantities of a solution of the antichlor at a time, testing the pulps, after allowing a few minutes for complete admixture after each addition. The test is made by immersing a piece of iodide of potassium and starch paper, which will be turned blue so long as any calcium hypochlorite is present. These papers are made as follows:— Three grms. of starch are ground up with a small quantity of water and poured into 700 cc. of boiling water, and to the solution 1 grm. of iodide of potassium and 0·5 grm. carbonate of soda are added. Sheets of white paper, water-leaf in preference, are now soaked in the solution and dried. In presence of calcium hypochlorite, iodine is liberated from the potassium iodide, and acting upon the starch forms with it a characteristic blue colour. _Loading._—It is usual, except in the case of papers of the very highest quality, to add to the pulp a quantity of some cheap loading material, such as china clay, or for the better {131} qualities of paper, “pearl-hardening,” or sulphate of lime. The addition of clay in moderate quantity can hardly be looked upon as an adulteration, since it serves to fill up the pores of the paper, and enables it to take a good surface in the subsequent operations of calendering. Moreover, it enables the manufacturer to meet the demand for cheap papers with some chance of remuneration to himself. If added largely, it of course tends to weaken the paper.