TOL. LXXX.] PHILOSOPHICAL TRANSACTIONS. CS& 



nating some above and others under. If however Government should find it 

 expedient to preserve the reference to proof spirit, from the tables given in this 

 report others may be constructed, in which all the old terms of over and under 

 proof should be retained, and have a precise meaning, as soon as the strength 

 to be called proof shall be finally settled. By the Act of 2 Geo. III. it is or- 

 dered, that the gallon of brandy or spirits of the strength of 1 to 6 under proof, 

 shall be taken and reckoned at 7 lb. 13 oz. which is understood by the trade to 

 mean at SS** of heat. Hence, taking the weight of a gallon of water at the 

 same heat to be 8 lb. 5.66 8cc. oz.,* the specific gravity of this diluted spirit 

 will be found .9335 at 60;'}- whence, by a computation founded on the tables 

 in this report, the specific gravity of proof spirit will come out .916. But the 

 rulers of correction belonging to Dicas's and Quin s hydrometers give the specific 

 gravity of proof spirits about .922 at 55°, equivalent to .920 at 6o^ The 

 former, .916, corresponds to a mixture of 100 parts of spirit with 62 by mea- 

 sure, or 75 by weight, of water; and the latter, .920, to a mixture of 100 

 parts of spirit and 66 by measure, or 80 by weight, of water. The difference 

 is considerable ; but the first is undoubtedly most conformable to the existing 

 acts of parliament. If therefore it be thought right to preserve the term proof- 

 spirit in our excise laws, it may be understoood to mean spirit, whose specific 

 gravity is .916, and which is composed of 100 parts of rectified spirit at .825, 

 and 62 parts of water by measure, or 75 by weight ; the whole at 60 degrees 

 of heat. 



Dr. B. has chosen this point of the thermometer, 6o°, in preference to 55° 

 because it is much the most suitable for experiments, being the temperature at 

 which a room feels pleasant, and in which any operation, however slow and 

 tedious, can be executed without the uneasy sensation of cold : for this reason 

 it has been adopted by many English philosophers. In the table formerly re- 

 commended, from 40 to 80 degrees of the thermometer, it will be the middle 

 temperature. 



The specific gravity of .825 having been fixed on as the standard of rectified 

 spirit in our tables, Mr. Gilpin was desired to ascertain by experiment what pro- 

 portion of water would be necessary, to reduce the lightest alcohol in his pos- 

 session to that standard. This was some of the alcohol which Mr. Lewis had 

 furnished; and its specific gravity being .8I4196 at 60", 3000 grains of it 

 mixed with 135 grains of distilled water formed a compound, whose specific 

 gravity was .825153 ; that is, in round numbers, 100 grains of alcohol at .814 

 with 4.5 grains of water, form our standard of spirit at .825. 



• Probably 8 lb. 5.7'^ oz. is nearer. — Grig. 



\ This specific gravity indicates a mixture of 107 grains of water with 100 of spirit, and conse* 

 quently is below Mr. Gilpin's present tables, which go only to equal parts.— Orig. 



