a comical question

i define a reference  in 2 ways
1.
<path id="Home.classpath">
     <pathelement location="bin" />
</path>
2.
<path id="${ant.project.name}.classpath">
     <pathelement location="bin" />
</path>
PS: ant.project.name = "Home"

in 1. way works well
in 2. way a error: Home.classpath not found

which different between 2 ways?