Vis is the power of Thaumcraft 2 and is used by its machines
There are several diffrent forms of vis that can be found
Liquid Vis
Liquid vis is the form of vis that is used by Thaumcraft machines and is generated from a Crucible.
Vis Crystals
Occasionally one can find natural crystals of pure Vis deep underground. These slowly grow over time into larger crystals. They grow much faster in areas where the aura has a positive charge. The crystals themselves have no effect on the local aura, unless the aura in the area becomes extremely low. In that case trace amounts of vis are released from the crystals which may cause the aura to replenish at a slow rate. Vis crystals have a myriad of uses which makes them a sought after resource. Care must be taken not to over-harvest them however, since doing so causes instability in the local aura which might lead to an increase in taint level. Without the crystals the aura also cannot replenish should it drop to low levels.
There are six types of crystals:
It is also possible to craft a placable version of the vis crystals if you have harvested enough of the individual crystals. As long as it is place on a solid surface it will grow and behave as a normal crystal.