Meselson and Stahl experiment

In this vodeo lecture Semiconservative model of DNA replication is Explained, Learn it at home and Send me Answers.
1. What was principle of this experiment?
2. What was conclusion of This experiment?
3. Which Nutrient was substituted in medium, To grow bacteria for this experiment? Why?

Link to Video: 
Semester: 
5
Unit: 
2
Subject: 
MICROBIOLOGY
Paper: 
1