1^8 On the Hydrometeri, 



Hencf it is evident tliat every variation of gravity in the 

 liquid will aflccl the indication on the hydrometer. 



This gravitv i? known bv the invm fpec'ific gravity ; and as 

 the Ipecific gravitv ot" Hqiiidi is not abfoUite, but relative, this 

 can only be determined bv comparifon : the lirft operation, 

 therefore, confiRs in getting one whofe abfoliite weight is 

 known, in order to ferve as a fixed point, and to determine, 

 in its turn, a comparifon of the denfity of the liquid. Pure 

 waier having always been confidered the beft ftandard for this 

 purpofe, the term fpeeiiic gravity is moft: readily defined, by 

 faying, that it is as the weight of the liquid is comparatively 

 with pure water: for inltance, if a phial, which holds pre- 

 cifcly looo grains of water, holds only 923 grains of a fpi- 

 rituoas liquor, the fpecific gravity of this fpirit is faid to be 

 922. 



But as fpirits expand by heat, and contraft by cold, this 

 fpeeiiic gravity varies according to the degree of the heat of 

 the liquor, which is called its temperature: therefore it is 

 neceCary firft to afcertain what that niav be, and it is done 

 very accurately by the thermometer. And in the experi- 

 ment mentioned, fuppofing the temperature to have been 55, 

 if the fpirit be heated to 70, the fpecific gravity will be di- 

 miniitied to 9151; aiid, if afterwards it be cooled down to 

 25 degrees of temperature, it will be found to have increafed 

 10 930^):. 



This is a very convenient manner of trying and exprefling 

 what the fpecific gravity of the liquor may be ; and as I Ihall 

 hereafter have occalion to ufc the term frequently, I hope I 

 fliall be excufed for explaining it in this place. 



All the variations of the fpecific gravity of fpirituous liquors, 

 and the caufes of them, may be comprifed under the follow- 

 ing heads, viz. 



ill, Thofe arifing from heat; and cold : sdly, A difference 

 in the proportions of the fpirit and water : and, 3dly, The 

 effects of penetration (or, as it is called, concentration), 

 which takes place on the mixture; all of which, being fo 

 clearlv defined by fir Charles Blagden, in his Report on this 

 fubjed, in the fecond part of the 8oih volume of the Tranf- 

 aclions of the Royal Society, 1 fliall beg leave to give it in 

 his words, and 'anient that the propnfed limits of this work 

 will not permit me to give the entire lleport, tverv paragraph 

 of vvhich I conceive to be important, and deinonilrates the 

 difficulties with which this fubjeft is enveloped, and very 

 clearly points out the way to remove them. 



*' Though (fays lir Charles) various indications of the 



ftrength^ 



