Subject: Why specifically is agar used as a bacterial growth medium?

I will be teaching high school chemistry in the fall, and I'm working with High 
School students this summer to inspire them to go into the sciences as a career.  
In an effor to inspire them, I try to answer ALL of their questions about the 
labs we do.  They also asked, "How did scientists discover that agar was the 
thing to use as a medium?"  Thank You very much.  David   (P.S.  Be as technical 
as you like.  I'll break it down for them.  Thanks.)

Re: Why specifically is agar used as a bacterial growth medium?