Does acetylene give a positive Tollens' test? If not, why is this so?
Tollen's reagent (ammoniacal silver nitrate) reacts with terminal acetylenes to form the silver acetylide which precipitates out of solution

$$\ce{RC#CH + AgNO3 + NH4OH -&gt; RC#CAg v + NH4NO3 + H2O}$$

In the case of acetylene itself, since both ends of the molecule are (if you will) terminal acetylenes, a bis-silver acetylide salt is formed and precipitates out of solution (see page 3, item 6b in a PDF from www.sakshieducation.com/ or via the Internet Archive)

$$\ce{HC#CH + 2AgNO3 + 2NH4OH -&gt; AgC#CAg v + 2NH4NO3 + 2H2O}$$

Tollen's reagent is not very stable, therefore it needs to be freshly prepared before use.  The reagent has a number of other uses in chemistry (test for aldehydes and &alpha;-hydroxy ketones), anatomical staining, and silver mirroring.  This Wikipedia article provides a nice, concise overview.