PBS Connections episode that traced the practice through ever advancing military tactics between adjoining kingdoms. Specifically, the tactic was the use of poison to eliminate a neighboring king so as to annex their land without war. Neighboring kings would presumably avert being poisoned when visiting their neighbors by pouring wine between their cups in turn. If one chose not to pour, touching glasses instead, then she or he would be exhibiting trust. (As a chemist, it was also fun to think of this as analytical chemistry in action, albeit somewhat rudimentary.) My conclusion, based on this foundation, has been that we should be at liberty to toast (and clink glasses) with any liquid as a sign of mutual trust.
Only problem with this, as Thomas Huxley said in a Presidential Address at the British Association in 1870, is that "the great tragedy of science [lies in] the slaying of a beautiful hypothesis by an ugly fact." In this case, if you check out snopes.com on the subject "Of Drinks and Clinks," they provide evidence (or lack thereof) that the story is entirely false. Instead, it appears that it emanates from the practice of communal drinking out of a common cup or vessel in numerous early cultures. While the question of poison does not enter this cup, the remaining ritual symbolically connects us to each other without the literal sharing of liquids (or germs!) Not surprisingly, I checked back to the PBS Connections episodes, and I can't find any discussion of wine; but more on Connections in a future post... The one good piece of news here is that the common cup did not necessarily cary alcoholic fluids.
So in my house, we will continue to toast over all liquids, including our morning espresso. Salud!