For converting the grain into flour, the inventors of the Nineteenth Century have made revolutionary changes. Milling processes within the last twenty-five years have been completely transformed by the introduction of the roller mill and middlings purifier. Formerly two horizontal disk-shaped stones or burrs were employed, the lower one stationary and the upper one revolving in a horizontal plane and crudely crushing the grain between them. In all modern mills these have been entirely displaced by porcelain rolls revolving on horizontal axes and crushing the grain between them. The first of these roller mills is shown in pat. No. 182,250, to Wegmann, Sept. 12, 1876. (See Fig. 164). The outer rolls _d e_ are pressed against the inner ones _a c_ by a system of weighted levers, and scrapers below remove the crushed grain from the periphery of the rolls. Many subsequent improvements have been made, one type of which employs a succession of rolls which act in pairs on the grain one after the other and reduce it by successive gradations. [Illustration: FIG. 165.--MIDDLINGS PURIFIER.] The _middlings purifier_, see Fig. 165, comprehends a flat bolt or shaker screen _b_, of bolting cloth, arranged as a horizontal partition in an enclosing case through which passes an upward draft of air produced by suction fan D at the top. This air passing up through the bolting screen lifts the bran specks and fuzz from the shaken material as it passes downward through the screen, brushes K being arranged below to keep the screen constantly clean. A representative and pioneer type of this machine is seen in Pat. No. 164,050 to George T. Smith, June 1, 1875, from which the view is taken. The useful effect of the roller mill and middlings purifier is to save the most nutritious and valuable part of the grain, which lies between the outer cuticle and the white starch within, and which breaks up in fine grains and is of a golden hue. This portion of the grain was formerly unseparated, and was mixed with the middlings and bran as an inferior product. Modern analysis has disclosed its superior food value, and the roller mill and middlings purifier have provided means by which it can be separated from the bran and incorporated with the flour, thereby greatly adding to its wholesome character and nutritive value, and imparting to the flour the rich creamy tint which characterizes all higher grades. Minneapolis, Minn., is the great center of the milling interests of the United States. The Pillsbury Mills are located there, and the “Pillsbury A.” which is said to be the largest in the world, has a capacity of 7,000 barrels per day. In 1877-78 disastrous flour dust explosions at Minneapolis brought about the development of the dust collector, for withdrawing from the air of the mills the suspended particles of flour dust, which not only invited explosion, but rendered the air unfit to breathe. Washburn’s Pat. No. 213,151, March 11, 1879, is an early example.