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 depends on the excise regulations, the limit being two per cent.; however, manufacturers of those non-intoxicating beers must remember that the spirit in them is an increasing quantity, for they always contain some yeast cells. When they leave the manufactory the beers may contain less than two per cent, of spirit, but by keeping the percentage may rise to three, four or even more per cent., unless thoroughly preserved, and such beer is liable to bear duty just as ordinary beers are. Every per cent, of sugar yields about a half per cent, of alcohol by fermentation, and as sugar is either contained in the materials from which small beers are made, or is freely added, it is easy to calculate the quantity of alcohol contained in such drinks. For every pound of sugar there will be half a pound of alcohol, which is equal to a pound of proof spirit. Thus, there would be two pounds of proof spirits to the gallon - a quantity which would produce a very exhilarating effect. A very thirsty man might make himself tipsy on such beer; but it must be said that there is very little beer of this strength sold, though no doubt some is made for private use.