Firestone Hy-N-Dry desiccator stopper 

Sintered-glass disc (porosity: 70 to 100 µm) sealed into bottom of stopper. When filled with 10 to 20 mesh drying agents, covered with plastic cap, and inserted into any ST joint vessel, becomes an inexpensive desiccator. Pinhole in cap allows assembled unit to "breathe" through, not around, desiccant with temperature fluctuations. Filled with Drierite^®, ST 24/40 stopper will absorb up to 1g of water. Plastic cap included.