The invention of the root beer float is often credited to Frank J. Wisner of Cripple Creek Colorado and dated to August 1893. However, an oft-repeated story about him having the idea for his "black cow" drink after seeing a full moon rise over a snowy mountain is probably apocryphal. Even if it isn't, nineteenth-century magazines de-bunk the idea that Wisner's idea was a novel one.
In 1892—a year earlier than Wisner supposedly imagined his black cow drink— The Western Druggist was already assuring its readers that, "The now quite general custom of serving ice cream with soda water is not of so recent origin as is commonly assumed." The publication goes on to explain that Eugene Rouselle had originated the idea in Philadelphia "some thirty-five years ago"—or around 1857. Rouselle, "who kept an elegant establishment on Chestnut street... Philadelphia, and who first introduced bottled soda in the United States" mixed chilled, flavored syrup with ice-cold cream and then added "a liberal quantity" of shaved ice. (He kept a carpenter's plane at his soda fountain to shave the ice.) He then dispensed carbonated water onto the creamy, sweetened mass of ice and served his customers "a most delicious and cooling drink." These were so popular that other purveyors of soda counters attempted to copy Rouselle's model, but his would-be rivals felt that his process was too troublesome: It used too much ice, and the cream would often sour. (The article doesn't specify whether the cream was souring because Rouselle's rivals weren't as conscientious about keeping it on ice as he was or because they were mixing it with more acidic syrups than Rouselle's. It seems likely that both problems were encountered at different times and by different businesses.) In order to make the process easier and more cost effective, many drug stores started replacing the combination of shaved ice and plain cream with a spoonful of ice cream, and the ice cream soda was born!
In August of 1891 the Hires root beer company was selling an average of 15,000 bottles of root beer extract per day and estimated that 1,500,000 glasses of their product were being consumed on a daily basis. Root beer syrup wasn't the only option for flavoring in ice cream sodas, but given the drink's beloved status the purveyor of a soda counter would have been extremely foolish not to stock it.
By 1897 ice cream sodas had taken over the soda trade so thoroughly that druggists were actively discouraging people from buying them in an effort to push their customers towards more profitable drinks. Soda dispenser D.W. Saxe wrote, "In hot weather room is valuable and time is money at the soda counter, therefore the foreseeing and level-headed dispenser will work every scheme possible to serve as many people possible in as short a space of time as he can... You can wait on ten customers for still drinks as quickly as one for Ice Cream Soda, and the per cent of profit is more than double... Now, then, is it not worth your while to push almost anything but Ice Cream Soda?" Saxe told his readers not to advertise ice cream soda "for at present Ice Cream Soda needs no advertising." Instead, he advised druggists to concentrate on advertising their still drinks and stated, "It is much easier now to make this change in your soda trade since nearly everybody, young and old, have taken to riding a wheel. Wheelmen of any experience whatever all know that Ice Cream Soda is not the proper drink when riding, and they want something to quench thirst and relieve that dryness of the throat and tongue. There is nothing better for this purpose than... Raspberry Cordials or Blood Orange Phosphates, and besides when you once get your customer educated to this style of drink he will want three or four of them in an evening while riding; whereas one glass of Ice Cream Soda and two glasses of ice water is the old rule."
Finding Saxe's commentary was particularly fortuitous. Gabriel actually doesn't like ice cream sodas, but he is definitely an avid Wheelman. (By this point in 2013 he had an antique 1887 high wheel bicycle—but more on that later.) I went out hunting for a blood orange phosphate, and found them in bottles in a cooler at our local bakery. Thus we were able to celebrate our holiday in thoroughly delicious style.
 P.B. The Western Druggist: A Journal of Pharmacy, Chemistry, and Allied Sciences. Volume XIV, 1892. p. 185.
 "Pepys Jr. in Philadelphia." The Illustrated American. Volume 7, No. 78. August 15, 1891. p. 628.
 Saxe, D.W. Saxe's New Guide or Hints to Soda Water Dispensers. Milwaukee: The Saxe Guide Publishing Co., 1897. pp. 10-14.