In old cocktail books, how much in mls is a glass of spirits?
A "wineglass" is typically interpreted as being 2 ounces or 60 ml.
glass, drink, or other similar strange measures are also assumed to be 2 ounces (60ml) or 1.5 ounces (45ml). Essentially what we today would refer to as a "shot". Imagine how confusing the measure of "shot" might seem a hundred years from now. At least "glass", "drink", and "wineglass" will almost certainly still have a connection to the craft, but how the heck does a gun fit in? :->
Perhaps the promotional material used for the Underberg digestif suggests an answer to where the word 'shot' comes from.
This product (made in Germany) is sometimes presented in the form of small, 20ml bottles covered in a brown paper wrapper. The top of this wrapper is twisted closed. Before the introduction of percussion cap bullets and carbines, measures of shot and gunpowder were distributed in similar little packets. The musketman would bite off the top of these little paper packets (which were greased with pig fat to keep out moisture - refer India Mutiny) and empty the contents down the muzzle of his rifle. This was followed by the packaging to form the wad. Whoever came up with this style of packaging for Underberg seems to have been making this link - even to the point that one of the point of sale devices was a bandolier set with loops for these little bottles that one could hang up as a display. Even today, your average rifle cartridge (say for a .303) holds about 20-30mls of liquid. Some glassware manufactuerers have also jumped on the bandwagon by producing 'shot' glasses that resemble shotgun cartridges.
Another derivation could be a medical link - to be given a 'shot in the arm' has the meaning to be stimulated to greater effort - as can happen when given an injection. In this instance 'shot' has the meaning of a non-specific 'dosage' (though one would assume something quite small as there is only so much liquid one can inject into a person's vein). This seems to be the angle taken by modern drinkers - one has a shot between longer drinks as some form of cure-all 'pep'.
Thank You Robert