In the 19th century, drinking vinegar as a tonic or restorative was considered good for your health. That itself wasn't a new idea - ancient Greeks drank oxymel, medieval Arabs drank sekanjabin and old Anglo-Saxons also drank oxymel. These were medicinal drinks that combined vinegar with sugar syrup or honey flavoured with herbs, spices, etc

As the syrup is diluted with water etc, perhaps soda water was added to make a "vinegar fizz'.

OXYMEL

1 pound raw filtered honey
1 cup apple cider vinegar

A basic oxymel can be made from honey combined with apple cider vinegar, gently simmered to a syrup.

SEKANJABIN 1

2 1/2 cups Water
4 cups Sugar
1 cup Wine Vinegar
1/2 cup Mint

Dissolve sugar in water; when it comes to a boil add wine vinegar. Simmer for 1/2 hour. Add the mint, stir, remove from fire, and let cool. Dilute the resulting syrup to taste with ice water (5 to 10 parts water to 1 part syrup). The syrup stores without refrigeration.
Some people replace the vinegar with lemon juice, but the shelflife of a lemon sekanjabin is about two months, whereas the vinegar seems to keep indefinitely.

SEKANJABIN 2

4 cups sugar
2 cups water
1 cup white vinegar
1/2 cup lemon juice
1 cup fresh mint leaves

In a heavy pan over medium heat, bring sugar and water to a boil. Boil for 5 minutes, then add vinegar and lemon juice and continue boiling until the syrup drips slowly from the end of a cold spoon. Lower the heat, add mint and simmer for 3 more minutes. Strain the syrup into a jar and let it cool. To serve, place an almond at the bottom of a glass, pour in 3 tbsp. of syrup, add ice water and stir. Garnish with a sprig of mint. You may also add a little grated cucumber to the glass.