OF THE AREOMETER, OR PROOF BOTTLE.
This instrument is indispensable to the distiller: it ascertains the value of his spirits, since it shows the result of their different degrees of concentration. I will give the theory of this useful instrument, as it may be acceptable to those who do not know it.
Bodies sink in fluids, in a compound ratio to the volume and the density of those fluids, which they displace. It is from that law of nature, that a ship sinks 20 feet in fresh water, while it sinks only about 18 feet in sea water, which has more density on account of the salt dissolved therein.
The reverse of this effect takes place in fluids lighter than water, as bodies floating in them sink the more, as the liquor has less density. Upon those principles are made two kinds of areometers—one for fluids denser than water; the other for those that are lighter: the first are called salt proof; the second spirit proof. Distilled water is the basis of those two scales: it is at the top for the salt proof, and at the bottom for the spirit proof; because the first is ascending, and the other descending; but by a useless singularity, the distilled water has been graduated at 10° for the spirit proof bottle, and at 0 for the salt proof. We shall only dwell upon the first, because it is the only one interesting to the distiller.
Water being graduated at 10° in the areometer, it results from thence that the spirit going to 20°, is in reality only 10° lighter than water; and the alcohol graduated at 35°, is only 25° above distilled water.
The areometer can only be just, when the atmosphere is temperate; that is, at 55° Fahrenheit, or 10° Reaumur. The variations in cold or heat influence liquors; they acquire density in the cold, and lose it in the heat: hence follows that the areometer does not sink enough in the winter, and sinks too much in the summer.
Naturalists have observed that variation, and regulated it. They have ascertained that 1° of heat above temperate, according to the scale of Reaumur, sinks the areometer 1/8 of a degree more; and that 1° less of heat, had the contrary effect: thus the heat being at 18° of Reaumur, the spirit marking 21° by the areometer, is really only at 20°. The cold being at 8° below temperate, the spirit marking only 19° by the areometer, is in reality at 20°. 2¼ of Fahrenheit corresponding to 1° of Reaumur, occasion in like manner a variation of 1/8 of a degree: thus, the heat being at 78½°, the spirit thus marking 21°, is only at 20; and the cold being at 87°, the spirit marking only 19° by the areometer, is in reality at 20°.
It is easily conceived, that extreme cold or extreme heat occasion important variations. For that reason, there are in Europe inspectors, whose duty it is to weigh spirits, particularly brandy: for that purpose they make use of the areometer and the thermometer. An areometer, to be good, must be proved with distilled water, at the temperature of 55°. Areometers, being made of glass, are brittle, and must be used with great care. This inconvenience might be remedied, by making them of silver; I have seen several of this metal. A good silversmith could easily make them; I invite those artists to attend to that branch of business; it might become valuable, as the distillers will be more enlightened.