Since the invention of the process of clay-coating paper, it has been found possible to incorporate in the coating solution certain materials which render it waterproof. The application of wax or paraffin to paper to make it waterproof is a common method; and although this product is largely used, the process has never reached the state of development expected. A great amount of time and money has been spent in the endeavor to perfect the process, and at the same time cheapen the cost, but so far with only indifferent success, and the experiment has usually proved very discouraging and expensive to its supporters. In its simplest form, this method consists in dipping the paper in a bath of melted paraffin, the paper being at a temperature lower than the melting-point of the paraffin, and promptly removing it from the bath, whereby the adhering paraffin is prevented from entering the paper to any considerable extent, and forms a thin coating upon its surface. This paper is odorless, and is used for wrapping meats, fish, butter, candies, etc. ♦Vegetable parchment♦ Parchment-paper, which is almost like the real parchment made from the skins of animals, is prepared from unsized rag-paper by immersing it for a few seconds in a solution of two parts of sulphuric acid, or oil of vitriol, in one part of water, at a temperature of 60° Fahrenheit, then washing it in cold water and removing any remaining traces of the acid by dipping it in a weak solution of ammonia. By this treatment it is rendered tough, translucent, glossy, and almost impervious to water. It is known as vegetable parchment, and is extensively used in wrapping lard, butter, meats, etc., and also to hermetically seal jars and pots for preserving fruit. ♦Grass-bleached♦ Silver tissue, or what is known to the trade as grass-bleached tissue, is extensively used for wrapping silverware. It is specially treated to remove all chemicals that would tend to corrode or tarnish silver. The best qualities of this paper are made in England. Metallic paper is paper washed with a solution of whiting, lime, and zinc. Characters written on this paper with a pewter pencil are almost indelible. ♦Litmus♦ Test or litmus papers are used in laboratories and factories for indicating the presence of acids or alkalies and various liquids. It is prepared by treatment with a peculiar coloring matter that gives its name to the paper. It is of a blue or yellow tint, according to the chemical employed in its preparation, and changes color under the influence of different chemical agents. The blue litmus paper, for instance, when thrust into an acid solution becomes red, but may be restored to its normal color by being subjected to the action of an alkali.