Are Tongues Biblical? Part 2

What was the purpose of tongues in the book of Acts? What were the rules for tongues when it was still in use? What is the “tongue of angels”? Does 1 Corinthians 14:4 teach a “private prayer language”? David compares tongues with prophecy in 1 Corinthians 14 and he explains 1 Corinthians 14:14-18.