Vista - Viewbox

Window x:Class="SimonFPlaybar.Window2"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window2" Height="300" Width="300">
<Grid>
<Viewbox x:Name="viewBox">
<TextBlock Text="Loading..." />
</Viewbox>
</Grid>
</Window>
Using the above XAML code shows 'Viewbox' on the screen and not
'Loading...' , Can any on advice me on my error.