In the preparation of $\ce{NH3}$ from $\ce{NH4Cl}$ and $\ce{Ba(OH)2}$, my teacher said that when heating the test tube, the test tube faces downward as opposed to upwards. Why is this so? Wouldn't the solid just fall out?

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/AvEVU.png