This section is from the book "Applied Science For Metal Workers", by William H. Dooley. Also available from Amazon: Applied Science For Metal Workers.
Most solid substances melt when a certain amount of heat is applied to them. Upon being heated further they vaporize. There are a few substances, like ammonium chloride, which vaporize without melting. To purify such substances, they must first be heated and their vapors collected. This process of purification is called sublimation.
 
Continue to: