Let's say that we have Na at room temperature. The concentration of Na becomes plus one percent due to impurities. I'm interested in finding the mean free path and collisional cross section due to these impurities. I'm not sure on how Matheissen's rule could be applied, if we don't know what the lattice mean free path and time are. Kittel does a horrible job in explaining this concept, and Ziman's text will probably get me scratching my head for the whole semester. Could some one also discuss some possibilities for the occurences of these impurities. Any suggestions would be much appreciated.