These are little envelops or tubes filled with a compound that absorbs moisture. I have placed one of these in each of my rod tubes and try to renew once a year. They seem to do an excellent job of keeping moisture away from my fine split cane rods. This liquid is available at your local hardware or paint store. Very little is needed to cause varnish to dry more than twice as fast.
Caution is needed here however.
I use a separate container of varnish mixed with Japan Dryer because the stuff is capable of solidifying an entire dip tube harder than cubic zirconium. When 35-mm slides are returned from the developer they are boxed in a small plastic case with a clear cover. This container is ideal for storing parts when stripping a bamboo fly rod prior to restoration. I place in these containers all guides, tip top, hook keeper, and wrapping thread removed from the rod. Since I started doing this I have yet to lose any parts.