The device on photograph is called French press it is used in coffee brewing to separate liquid from coffee particles (small ones can be pushed through filter by pressure of pouring coffee), it also makes pouring coffee easier (otherwise particles will block a big part of the filter).
And yes, its useless for tea, but adds kinda "european kick".
The solubility of caffeine at 25C is about 16 mg/ml http://www.sigmaaldrich.com/etc/... , so 200ml would be able to hold up to about 3200 mg, or 3 grams.
I have personally studied this problem, and the solubility of caffeine is not the rate limiting step for dissolution when steeping in a cold brew scenario. There are a couple factors related to why caf... (more)