The photogelatin process is well adapted to the reproduction of paleontologic drawings, wash drawings, photographs, photomicrographs, works of art, old manuscripts--in fact, any kind of subject in which the reproduction of delicate lights and shades is essential. If properly manipulated it has distinct advantages over the half-tone process in that it can reproduce details and light and shade without showing the effect of a screen and without the use of coated paper. Excellent reproductions by the heliotype process are also made in color by first printing the design in a neutral tone and superposing appropriate transparent colors on this print, somewhat as in chromolithography, so that the colors softly blend with the shaded groundwork. Reproductions made by the photogelatin process are more expensive than those made by the half-tone process, for the prints are generally made on better paper and are printed with greater care. They give no screen effect and are perhaps unrivaled by prints obtained by any other process except photogravure, in which the image is printed from a metal plate that has been sensitized, exposed under a reversed negative, and etched. Changes can not be made on photogelatin plates except by making over the corrected parts. All retouching must be done on the originals or on the negatives made from then. LITHOGRAPHY. ORIGINAL PROCESS. The general term "lithography" is sometimes used to indicate not only the original process so named, said to have been invented by Senefelder, but chromolithography, photolithography, and engraving on stone, as well as engraving on copper as a means of supplying matter to be transferred to and printed from a lithographic stone. Senefelder discovered that limestone will absorb either grease or Water, and that neither one will penetrate a part of the surface previously affected by the other. He found that if a design is drawn on limestone with a greasy crayon and the stone afterward properly prepared with a solution of nitric acid and gum, greasy ink will adhere only to the parts that are covered with the crayon, and that the stone will give off an impression of the design. Lithographic stone is described as a fine, compact, homogeneous limestone, which may be either a pure carbonate of lime or dolomitic--that is, it may contain magnesium. Although limestone is one of the most common rocks, limestone of a quality suitable for use in lithography is found at only a few localities.[11] There are two general classes of lithographic stone, known to the trade as "blue" or hard stone and "yellow" or soft stone. The blue stone is adapted for engraving and to the better grade of fine-line printing; the yellow stone is rated as somewhat inferior. [Footnote 11: Kubel, S. J., Lithographic stone: U. S. Geol. Survey Mineral Resources, 1900, pp. 869-873, 1901.]