As it reaches the required fineness, then, the crushed rock escapes from the stamp machine and flows away in the stream of water, and although much gold is caught by the mercury, it is by no means all. The stream is therefore directed over tables formed of copper sheets coated with mercury, so that additional opportunities are given to mercury to catch the grains of gold. Moreover, the table, which, by the way, is placed at a slight incline, is broken at intervals by little troughs of mercury called riffles, which assist in the depositing and catching of the metal particles. But even then all the gold is not captured. The crushed rock is now like sand, and some of the grains still contain gold, which has not been detached by the crushing. The gold, however, makes such grains slightly heavier than the others, and because of that they can be separated. The old way is to use a blanket table, a table, that is, covered with coarse flannel or baize, the hairs of which catch these heavier particles as the water stream carries them along, the lighter particles escaping. The grains so caught form what are known as "concentrates," since in them the gold is concentrated. The concentrates are subsequently treated as we shall see later. Now we can see how modern scientific methods have supplemented the old ways. Take first the case of the stamp mill or stamp battery. In spite of that prime virtue of simplicity which has kept it at work almost unchanged for centuries, it has its weaknesses, and no doubt for some purposes crushing mills are better. Of these there are a great variety, several of which depend for their action upon centrifugal force, or, as it is more correctly termed, "centrifugal tendency." In these crushing mills there is a ring, generally of steel, inside which are suspended one or more heavy iron rollers. The shafts which carry these rollers are attached by their upper ends to the driving mechanism on the top of the mill, and when that is set in motion the rolls are carried round and round inside the ring. Because of the centrifugal tendency, they swing outwards, pressing heavily against the inner surface of the ring. The rock is fed in in such a way that the rollers, as they roll round the inside of the ring, repeatedly travel over it and crush it. In another type of mill, called the ball mill, the principle is different. There you have a cylinder of steel which turns upon a horizontal axis. This cylinder is partly filled with steel balls of various sizes, and as the mill turns, the rock, being mixed with these balls, is pounded and broken up. As the mill turns over and over the balls fall upon the pieces of rock, thus producing a fine powder. Other mills, again, are but refined editions of the common mortar mill so often seen where building operations are going on, in which heavy iron rollers travel over the material to be crushed as it lies in a round pan.