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.
A manufacturer of carbonated beverages, desirous to do well and economize money, should always manufacture his own syrups, as then only will he be certain of what he is doing. The preparation should always be made as short a time before use as possible. The strength of the syrup for the manufacture of saccharine beverages is generally from 25° to 32° Baume. The syrup should never be made too dense, as it is apt to deposit some crystals of sugar during cold weather. Many prefer weak syrup, as being more liquid and more easily gauged by the syrup gauge. Ten to twelve pounds of sugar to the United States gallon of water (eight pounds) or twelve to fourteen pounds to the Imperial gallon of water (ten pounds) is the usual proportion employed, which of course can be varied to suit circumstances.
If a syrup of 32° has been prepared, and occasion demands a weaker one, it can be reduced to any desired strength by the addition of water, which should be thoroughly agitated or mixed with the syrup.
Syrups are judged by the laboratory man to be sufficiently prepared when some, taken up in a spoon, pours out like oil, or a drop cooled on the thumb-nail gives a proper "thread" when touched. When a thin skin appears on blowing upon the ryrup, it is judged by the same party to be completely saturated. These rude tests often lead to errors, which might be easily prevented by employing the proper proportions, or determining the specific gravity.