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.
This means of producing foam on bot tied beverages we do not recommend at all, as it will decompose in time, and make the syrup, and consequenly the beverage, taste and smell of sulphuretted hydrogen, suggesting that of rotten eggs; however, if desired, it may be prepared in small quantities only for the dispensing counter. There it shows an attractive-looking white foam. Prepare as follows: take the white of a few eggs, and some powdered sugar, beat well together and add to syrup.
Whatever kind of gum foam is employed, it should be used but sparingly, as the customer does not like to get a glass of foam instead of the expected refreshing drink. Where plenty of sugar coloring has been used, as in sarsaparilla, root beer, etc., it is not necessary to add gum foam, as the former produces foam enough; but if desired to retain a beat, some gum foam may be used.