The practical operations necessary for the preparation of the size are comparatively simple. In the case of size containing relatively small percentages of free rosin, the boiling is conducted in open vessels, but for the manufacture of rosin size containing large proportions of free rosin boiling under pressure in closed vessels must be resorted to. With the open pan process a steam jacketed pan is used, and the required quantity of alkali, dissolved in water, is placed therein and heated to boiling point. The rosin well powdered is added in small quantities from time to time, this being effected cautiously in order that the carbonic acid gas set free during the process may readily escape. The rosin is generally completely saponified after four or five hours' boiling. It is then passed through strainers into store tanks, from which it is drawn into the beating engines as required. In the case of rosin boiled under pressure a cylindrical vessel provided with a manhole at the top is used. The correct amounts of alkali and water are put into the digester, and also the rosin in a powdered form, the digester being fitted with a perforated plate placed about two feet above the bottom of the vessel in order to prevent the rosin forming into a hard mass at the bottom of the digester. It is possible in this way to manufacture a thick size containing 30 or 40 per cent. of free rosin and a comparatively small proportion of water. Many paper mill firms prefer to purchase such size ready made. The most recent modification of the ordinary rosin size is a compound prepared by treating rosin with silicate of soda. This alkali dissolves rosin readily, and the soap obtained when suitably diluted with water decomposes in the beating engine on the addition of aluminium sulphate, with the precipitation of a gelatinous silica which assists in hardening the paper. Bacon has patented a process in which powdered rosin is melted down with dry crystalline silicate of soda. The resultant product is ground to a fine powder, which is then ready for use. It dissolves easily in water, and when decomposed with the proper proportion of alum gives a gelatinous viscous mass said to have excellent sizing properties. The advantages of a dry powdered rosin size readily soluble in water are obvious. _Loading._--The term "loading" is applied to the various substances which are employed for the purpose, as it is commonly supposed, of making paper heavy. But china clay and similar materials are not added simply in order to give weight to the paper, since they serve to produce opacity and to improve the surface of papers which could not be satisfactorily made unless such materials were used.