Many paper-makers add to the gelatine a certain quantity of soap, the effect of which is to render the paper capable of taking a high finish. The soaps employed should be white and firm, and should be free from rosin. Some soap-makers prepare a special soap for paper-making. They are usually made from tallow, or a mixture of this with a small quantity of coconut oil. If a solution of soap be added to one of alum, a double decomposition immediately occurs, the fatty acid being thrown down in combination with the alumina, and the soda {164} combining with the sulphuric acid. If, however, the soap solution be previously mixed with a solution of gelatine, no precipitation takes place, the mixture forming an emulsion having somewhat the appearance of thin milk. Various attempts have from time to time been made to size paper in the engine with gelatine, by precipitating it in the pulp after the manner of rosin sizing, but as yet no successful method has been devised. _Single-cylinder Machines._—Modified forms of the original Fourdrinier machine have been introduced to meet various requirements. One suitable for the manufacture of very thin paper, or of papers one side only of which is required to be glazed, called a single-cylinder or Yankee machine, is shown in Fig. 56. [Illustration: FIG. 56.] It resembles the Fourdrinier machine as far as the couch-rolls A and B. The paper is taken off the wire-cloth on to an endless felt running round the upper couch-roll A, and travelling in the direction of the arrows. It is taken from the felt on to the large drying-cylinder C, of about 10 feet diameter, heated with steam. It is carefully turned and polished so as to impart a high gloss to the surface of the paper with which it is in contact. Calender-rolls are sometimes supplied in addition. The arrangement shown at D is for the purpose of washing the felt. This is necessary to cool and open it out after passing between the cold press-roll E and the hot cylinder. {165} The paper, after passing over the greater part of the surface of the cylinder, is sufficiently dried, and it is then wound off at F. [Illustration: FIG. 57.]