The arcanist can learn to cast a spell by suffering from its effects. When the arcanist fails a saving throw against a spell cast by an enemy, as an immediate action she can expend 1 point from her arcane reservoir to temporarily acquire the spell. She can cast the spell using her spell slots as if it was a spell she had prepared that day. The spell must be on the sorcerer/wizard spell list and must be of a level that she can cast. The ability to cast this spell remains for a number of rounds equal to the arcanist’s Charisma modifier (minimum 1).
Pathfinder Roleplaying Game: Advanced Class Guide © 2014, Paizo Inc.; Authors: Dennis Baker, Ross Byers, Jesse Benner, Savannah Broadway, Jason Bulmahn, Jim Groves, Tim Hitchcock, Tracy Hurley, Jonathan H. Keith, Will McCardell, Dale C. McCoy, Jr., Tom Phillips, Stephen Radney-MacFarland, Thomas M. Reid, Sean K Reynolds, Tork Shaw, Owen K.C. Stephens, and Russ Taylor.