In the early part of the 20th century, soda fountains were all the rage. In addition to a place to get tasty refreshments while shopping, a store's soda fountain was a novelty and a meeting place. Carbonated drinks were mixed up fresh behind the counter, and all kinds of innovative recipes were promoted. But soda pop and ice cream floats weren't all that popular in cold weather, so soda fountains came up with creative hot drinks. Oh sure, there was coffee, tea, and hot cocoa, but also soup and proprietary recipes that combined soda with bizarre ingredients like eggs, bouillon, or clam juice.
The popularity of hot drinks didn't happen on its own. Trade magazines and books not only published recipes, which they often called formulas, for everything from hot pineapple juice to hot malted orange, but they also offered promotional ideas and sales tips. An issue of The Soda Fountain, for example, suggested staging a "Hot Soda Pageant This Winter." The editors proposed selecting different drinks each month and promoting them with window displays. One month would promote hot milk and egg drinks; another bouillons, broths, and soups; another coffee, tea, and chocolate drinks; another hot fruit drinks. During malted-milk-drinks months, they recommended a "peaceful scene with cows grazing, milkmaids, great pails of foaming milk and happy, healthy youngsters in the foreground."
Read about the era of "hot sodas," and find recipes for Hot Cherry Egg Bounce, Hot Egg Lime Juice Fizz, and Reeking Smatch at Atlas Obscura.