An Ice Cream Soda is a foamy, non-alcoholic drink made with ice cream that ends up as a fizzy slurry drink.
The foaminess is a result of the ice cream interacting with the soda water. You don’t mix either of them to make a harmonious blend, instead, you want a contrast in tastes and textures.
There are several methods of making an Ice Cream Soda, but they all involve actual soda water, which both takes the edge off the sweetness of the drink, and makes it foamy.
The ingredients are plain soda water, flavoured syrup, a small amount of cream or milk, and ice cream. You put the syrup and the cream in the glass, stir it, add some soda water, stir to blend, then fill the glass two-thirds or so with more soda water, then finally a scoop of ice cream.
Another method is to put some syrup in a glass with a small amount of ice cream, mix together until the ice cream was soft and blended with the syrup, then fill with soda water and then top with more ice cream.
At soda fountains, the technique is to put two or so tablespoons of a flavouring syrup in the bottom of a tall glass, and then jet in 2 or 3 oz (30 to 90 ml) of soda water. You then add a small amount of ice cream, and then jet the ice cream with more soda water so that it will liquidize some of the ice cream. Then you add more syrup, then more ice cream, then toppings which can include whipped cream.
At soda fountains, the water comes right out of the “soda fountain”, an actual tap that just dispenses soda water. Pulling the handle towards you causes the water to flow out normally; pushing it away from you causes it to come out in a high-pressure jet of water. The syrups is generally in pumps off to the side.
Ice Cream Sodas need to be drunk relatively soon after being made. You can’t make them ahead; they are made to order.
They are served with both a straw and a long-handled spoon. Traditionally, the straw was one that came wrapped in paper, stuck to the side of the glass, the condensation holding it in place. The spoon would already be in the glass. If you were sharing, you could get two spoons and two straws.
Special Ice Cream Soda glasses are wider at the top than the bottom. You can start by drinking them through the straw, or eating the ice cream and toppings with the spoon.
Various combos have names such as The Hoboken (made with chocolate ice cream, topped with whipped cream, pineapple syrup and a maraschino cherry.)
Many places these days make Ice Cream Floats and pass them off as Ice Cream Sodas. But purists say the real thing has to have soda water in it, not just carbonated water, or pop, and not a flavoured soda pop replacing the soda water and syrup.
“The only firm of which we know anything whose business it is to concoct drinks which can compete on equal and even superior terms with those that inebriate, is that of the Messrs. Dows, CLARK, and VAN WINKLE, who introduced with such marked success iced cream soda-water at the great Paris Exhibition [Ed: of 1867.] Now, if the teetotalers want to run a powerful opposition to spirits, wines, beer, and all the paraphernalia of the gin-palace, they can most effectually do so through the machines used by that firm. They contain a draught apparatus, cans with any selected syrups, 4, 6, or 8; a cutter for reducing ice to raspings resembling snow, and an arrangement for mixing cream.
The taps are so designed that they froth up the liquid in the glass at the discretion of the drawer, coming out with more than the creamy foam of the best ale. The soda is so prepared that it retains its effervescing qualities in the glass, not bubbling ever while the gas escapes in drawing. This gives a peculiarly grateful sharpness to the drink, in contradistinction to the flatness of soda-water left open. Through the creaming effervescing liquid comes out the raspberry, vanilla, lemon, nectar, pineapple, or any other flavour of the syrup, making a drink which has all the exhilarating qualities of the very best alcoholic beverages, without their reactionary and deleterious effect, as nourishing as it is refreshing and agreeable.
Indeed, it is almost impossible to conceive anything more pleasant on a hot day than a draught of the iced-cream soda-water, and were the machine used as it ought to be, a really effective opposition would be introduced. Indeed, publicans would find the benefit of running these beverages against their others, for they do not desire to poison mankind, but to make a profit. Every Club-house should have one, every school; indeed we go further, and say every house containing a large family. In each regimental mess and canteen they ought, of course, to be ; also on board every ship; and for winter as well as summer, for although not so useful in cold weather, they can be modified to supply hot sodawater drinks. Though these machines look very formidable, being made of marble and supplied with numerous taps, and are rather expensive for a family, ranging from £ 7 or £8 up to £40 or £50, they would save money wherever people congregate.
Large cylinders of aerated water are supplied, its manufacture being retained to prevent adulteration, or inferior waters being used by those who retail the iced-cream soda. The fact comes conspicuously out that spirits are not required to give flavour, or even exhilaration, that a sharp, strong, exciting, and delicious draught can be concocted more cooling than an an ice cream, and as exhilarating as champagne..” — Author unidentified. Drinks column in Anglo American Times. London. 25 March 1871. Page 3.
[Ed: Dows was Gustavus D. Dows of Lowell, Massachusetts. He was a retail druggist. The firm referred to was headquartered in Boston.] 
Literature & Lore
MONEY IN FIZZ. The New Castle Soda Fountains Reap Big Harvests of Coin
People who look upon the seductive water here no doubt noticed that New Castle druggists are charging 10 cents this year, when that effervescent beverage contains ice cream. This is an advance of five cents over last year. A druggist, in giving his reasons to a NEWS reporter for this increase, said:
“Last year there was not as much money in ice cream soda at five cents as there should have been, because we had to give crushed fruits. We are charging ten cents this year, but we are giving twice as much ice cream. To make good ice cream, as some of the druggists do, it costs pretty near $1 a gallon. Fruits cost twice as much as plain syrups. Phosphates, root beer soda without ice cream, etc., yet remain at five cents.”
There must be money in the business hereabouts. It is said that a certain Washington street druggist sold 23 gallons of ice cream in a week in soda water. He made enough money out of his fountain to pay his clerk hire and rent for a year. It is estimated that this druggist sold 35,000 glasses of soda water during the season. There are about 10 fountains in New Castle, not including the small vendors, and if the others did as big business as the druggist mentioned the consumption of soda water in New Castle last summer must have amounted to some 350,000 glasses.
— The New Castle News. New Castle, Pennsylvania. Friday, 24 April 1896. Page 1.
In Chicago, asking for a “soda” used to mean an “Ice Cream Soda.” If you wanted “soda pop”, you asked for “pop.”
 Chemist and druggist: the newsweekly for pharmacy, Volume 35. 28 December 1889.