Seder 117: On the Placement and Timing of Numbers 19

 In the religion of ancient Israel, one of the most serious kinds of ritual impurity resulted from contact with a human corpse.  While contact with a dead animal resulted in a minor impurity (Lev 11:31, 39) that only lasted "until the evening," contact with a human corpse was more serious.  During Israel's time in the wilderness, those who had had such contact were temporarily excluded from the camp (Num 5:2).  The high priest was to avoid it, even for the sake of close relatives (Lev 21:10-11).  It prevented a person from participating in Passover at the usual time (Num 9:6).  One who encountered a corpse during a Nazirite vow had to start the vow over again after a cleansing period of a week (Num 6:9-12).  

The ritual procedure for dealing with corpse impurity is laid out in Numbers 19.  It involves the application of a special mixture of living water and ashes of a red heifer on days 3 and 7 of a 7-day period.   

There seem to be allusions to this procedure in the Torah prior to Numbers 19.  The fact that a Nazirite's cleansing from corpse impurity requires a week (Num 6:9) is one.  When the Levites were set apart for service, the "water of purification" was sprinkled on them.  Presumably this is the mixture described in Numbers 19.  These allusions imply that the procedure in Numbers 19 was revealed to Moses at Mount Sinai and used throughout Israel's time in the wilderness.   

The placement of the description of this procedure in Numbers 19, rather than in Leviticus 11-15 or earlier in Numbers, seems to be designed to communicate a message.  In the book of Numbers, chapter 19 is located between the story of the first generation of the Exodus (up through year 39 in the wilderness) and the story of the new generation that completes the journey to the Promised Land.  Many people died during those years in the wilderness, but the nation was cleansed and renewed in preparation for the completion of the journey.