Half an ounce of boric acid, and the promise that it will keep indefinitely.
Mix six pounds of sugar, two quarts of water, four ounces of citric acid, one half ounce boric acid. Dissolve over a slow fire. When cold add sixty ounces refined lime juice, four ounces tincture of lemon peel, and enough water to make two gallons. This will keep indefinitely.
On Uncle Sam's Water Wagon, 1919. Public domain; transcription unaltered.
Boric acid and its salt borax were standard commercial preservatives around 1900, and they are now barred from food in the United States, the EU and most other places. They accumulate, and they were used everywhere.
The syrup was mostly safe already — that much sugar and citric acid is hostile to spoilage. The boric acid is insurance for an opened jug standing in a warm room.
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The 20th Century Home Cook Book · 1906
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Mackenzie's Ten Thousand Receipts · 1867