A few more words might be said on the subject of distillation. I am sometimes asked to explain more fully the term "destructive distillation." When a complex substance like wood or coal is heated some of its ingredients are made volatile at the high temperature, and so escape as gases. The wood itself is broken up into simpler substances. It is plain that in this process the original substance is lost as such, new substances taking its place, and we therefore speak of the process as destructive distillation.

When water containing various salts or gases in solution is heated, the gases will be given off as the temperature rises. At the boiling point, the water itself will begin to pass off as vapor. The salts will not vaporize unless much more strongly heated. If the steam be collected and cooled, it will condense to form pure water. This in an illustration of simple distillation. If a mixture of alcohol and water be heated some of the alcohol will vaporize before the water. It may in this way be separated from the water, and this process is called fractional distillation. This is the principle employed in the manufacture of whiskey, etc.