The Vis condenser provides another way of gaining Vis. By placing any kind of vis crystal in the slot on the left the condenser uses it as a catalyst to slowly draw pure vis from the aura. This has the unfortunate side effect of corrupting the crystal however and after some time it will turn into a tainted crystal. Unlike the crucible however, the condenser produces 100% pure, untainted vis.
The speed at which the Condenser draws vis from the atmosphere is determined by the current phase of the moon. The fuller the moon is, the faster the condenser works. More importantly, the amount of vis that can be drawn with a single crystal also varies with the phase of the moon - the fuller it is, the more vis can be drawn before it is depleted.
It is possible to place condensers next to each other and they distribute their vis amongst each other so you don't have to connect them all to conduits.
Using a Quicksilver Core will increase the Speed of the Condenser, while using a Stabilized Singularity will increase the Efficiency. Using Concentrated Evil will allow you to use Tainted Crystals, but this will produce Taint.
Source - Thaumcraft2 Wiki