Tractate Tahorot - Chapter 1 - Mishnah 6

An egg's worth of food that has second degree uncleanness and an egg's worth of food that has third decree uncleanness that were mixed together are regarded as having second degree uncleanness. If they were then divided, each part is regarded as having only third degree uncleanness. If each part separately fell on a loaf of terumah they do not render it invalid. But if the two fell together they convey to it third degree uncleanness. An egg's worth of food that has first degree uncleanness and an egg's worth of food that has third degree uncleanness that were mixed together are regarded as having first degree uncleanness. If they were then divided, each part is regarded as having only second grade uncleanness, for even the third grade that touched the first has become only a second grade. If two eggs worth of food that have first degree uncleanness and two eggs worth of food that have second degree uncleanness were mixed together they are regarded as having first degree uncleanness. If they were then divided, each part is still regarded as having first degree uncleanness. But if they were divided into three or four parts, each is regarded as having second grade. If two eggs worth of food having second degree uncleanness and two eggs worth of food having third degree uncleanness were mixed together, they are regarded as having second degree uncleanness. If they were then divided, each part is still regarded as having second degree uncleanness. But if they were divided into three or four parts, each is regarded as having only third degree uncleanness.