CHANGING THE PERCENTAGE OF ALCOHOL. 115 



of the scale. In absolute alcohol it sinks to 100, the upper end. 

 Mixtures of the two liquids permit it to sink to various depths, and 

 the number corresponding with the surface of the liquid indicates 

 the percentage of alcohol by volume. 



A. Alcohol that has been used for the preservation of specimens usually holds in solu- 

 tion or suspension substances of greater specific gravity than alcohol or even water. Their 

 presence increases the specific gravity of the liquid, and causes the per cent, of alcohol to 

 appear less than it really is. Old alcohol should therefore be cleared (§ 294), if necessary, 

 before testing vs^ith the alcoometer ; but filtration will not, of course, remove materials 

 which are in real solution. 



B. As stated by Giinther (§ A, 697), the British hydrometer is so arranged that the 

 zero corresponds with proof spirit, about 49 per cent. The other grades are designated as 

 so many degrees above or below proof, and two degrees equal but one per cent. For 

 example, our 95 per cent, alciihol would be 92 above proof, while 20 below proof would 

 indicate the presence of 39 per cent, of absolute alcohol. 



§ 269. Hijdrometer Jar. — This is a tall and narrow glass jar, 

 mounted on a foot. The alcohol to be tested is poured into it, and 

 the scale may be read through the glass. 



Any glass jar of sutficient height (c. g. the 3 x 10 in. jar of Whitall, Tatum £; Co , A), 

 will answer in using the alcoometer, but the narrowness of the proper hydrometer jar 

 requires a less quantity of the liquid, and the scale is read more easily. A cylindrical 

 graduate on a foot, and holding about 500 cc, makes an excellent hydrometer jar, but is 

 more expensive. Finally, a cheap one may be made by corking one end of a tali lamp 

 chimney or piece of large glass tubing, and fixing it into a hole in a wooden disk. 



Unless care is sure to be observed iu introducing the alcoometer, a piece of soft sponge 

 or some cotton should be pushed to the bottom of the jar. 



§ 270. Changing the Percentage of Absolute Alcohol in a 



Mixture. — This may be done by mixing two grades of alcohol, or 

 by adding water to one of them. The due proportions may be 

 ascertained either by experiment or by the arithmetical method 

 known as alligation alternate. 



§ 271. Rule of Alligation Alternate. — Find the difference be- 

 tween the required per cent, and the per cent, of each of the liquids 

 to be combined. AVrite the reciprocal of each of these different 

 numbers, and reduce the fractions to a common denominator. The 

 numerators will then represent the proportionate volumes of the 

 two liquids. 



Examples. — Water and 95 per cent, alcohol are to be so combined that the per cent, of 

 the mixture will be 50 ; 95 — 50 = 45 ; 50 — CO (the water) = 50. The reciprocals of the 

 two numbers are J^ and ^iy ; reduced to a common denominator, these fractions become 

 tIo ^nd /JL respectively; hence 9 volumes of water are to be added to 10 volumes of alco- 

 hol. On the alcoometer the per cent, of the mixture will be indicated as about 55. 



Again, two grades of alcohol, respectively 75 and 30 per cent., are to be combined so 



