What are the impure parts of a dog's body, and how is ritual purity from its impurity achieved in detail?

Scholars have differed regarding the impurity of a dog, and the method of purifying what it touches is by washing it seven times, one of them with earth. They have also differed as to whether something other than earth, such as potash or soap, can take its place. What appears most probable is that earth is specifically designated due to the textual evidence, and this is supported by modern medicine, which has proven that a dog's saliva contains germs that are not removed except by washing seven times, one of them with earth.