The Arculus method is a common arrangement of equipment used for passing steam over magnesium and for the cracking hydrocarbons or alcohols.
Details can be found here
Solid reagents can simply be placed in the bottom of the tube. Liquid reagents are absorbed onto an inert medium.
Higher up the tube is placed the other reagent or catalyst
The tube is placed almost horizontally so as to stop the reagents mixing.
When heat is applied, the substance at the bottom of the tube vapourises (or gives off a gas) and this passes over the heated second reagent.
Variations of the technique can be used for:
1) Reaction of metals with steam
2) Reaction of metals with oxygen
3) Dehydration of alcohols
4) Cracking of liquid paraffin
