Glyphicon inside button not visible

Question

I would like to put glyphicon inside button. I don't know why it's not visible. The only thing that is visible is text, but not glyphicon.

Here is example. Problem is with this button:

<button type="button" class="btn btn-primary">
<span class="glyphicon glyphicon-arrow-up"></span>3</button>

Answer 1

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">

<button type="button" class="btn btn-primary">
<span class="glyphicon glyphicon-arrow-up"></span> 3 </button>