This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
We append an approved formula:
Ginger root, first quality, ground, fifteen ounces; orange peel, sliced and cut, six ounces; nutmeg, one and one-half ounces; vanilla beans, Mexican, six drachms; cinnamon (Ceylon), one and one-half ounces; alcohol 95°, sufficient. Capsicum if desired: try from fifteen grains and upwards, until it suits the taste
Select the best of nutmegs and bruise them in an iron mortar, the vanilla beans slice and cut into small pieces; the cinnamon also bruise in a mortar. Mix all the powders together, moisten with eight ounces of alcohol of 95 per cent, and pack firmly in a percolator; then add enough alcohol of the same strength to perfectly saturate the powders and leave a stratum of alcohol above it. Cover carefully and macerate for forty-eight hours. Then allow the percolation to proceed, gradually adding more alcohol until twenty fluid ounces of extract are obtained (or more if desired).
Another method is to prepare this extract by digestion. Add to the percolate about two drachms of essence of oenanthic ether, and about one drachm of essence of rose. When lime juice is preferred to be added, only the purest kind should be used; about an ounce or so will be the proper amount. Prepare this extract for months in advance; keep air-tight, closed, in a cool place, and the aroma of the extract will considerably improve by age. When it is wanted for use, treat it with distilled water and pure pumice or silica as directed before, in order to make it water soluble. A chief feature in preparing a good Belfast ginger ale is also to mix this soluble extract thoroughly with the syrup. The syrup and flavor should be allowed to combine for twelve hours in covered earthenware vessels, occasionally mixing with wooden (no metallic) spatula, in order to attain a perfect diffusion of the flavor with the syrup, which is eminently important for the success and production of high-class beverage. The proportions of this formula may be altered to suit.