SPECIFIC DENSITY 



615 



mark, and weighed ; the whole is then emptied 

 and tilled with water alone up to the mark, and 

 again weighed. The two weighings give the data 

 for ascertaining the ratio between the weight of 

 the solid and that of an equal bulk of water. If 

 the solid is acted upon by water, some other liquid 

 of known specific density must be employed, and 

 the calculation varied accordingly. If it be lighter 

 than water, it is coupled with a piece of heavy 

 substance whose weignt and specific density are 

 separately known, and the aggregate apparent loss 

 of weight incurred by the combination on being 

 immersed in liquid is found by the hydrostatic 

 balance. Of this aggregate so much is due to the 

 heavy substance and the remainder to the light 

 solid. This gives data for calculating the specific 

 density of the light solid. The specific density of 

 a liquid is ascertained by simply comparing the 

 weiglits of quantities of that liquid and of water 

 successively made to fill the specific gravity flask 

 np to the same marked level ; or by comparing the 

 apparent losses of weight incurred by a solid on 

 being immersed in water and in the liquid respect- 

 ively ; or by the use of hydrometers or areometers. 

 The areometer (araios, 'thin,' and metreo, 'I 

 measure ; ' Fr. artom&tre or ptee-liqueur ; Ger. 

 Araometer or Senkwaye) or hydrometer is a gradu- 

 ated instniment which floats in a liquid, without 

 being wholly submerged, under the equilibrium of 

 the weight of the whole body acting downwards, 

 and the buoyancy of the liquid, equal to the weight 

 of the part of the liquid displaced, and acting up- 

 wards. The specific density of a uniform cylinder, 

 Bay of ice, floating vertically in water U the volume 

 immersed -r the whole volume ; and in liquids of 

 different specific densities such cylinders would 

 sink to different depths. But it is more convenient 

 to use graduated hollow glass instruments weighted 

 with mercury at one end to make them float verti- 

 cally (see fig. 1 ). AB is graduated ; C is a 

 large bulb ; D is a small bulb containing 

 mercury, the quantity of which is so adjusted 

 that the instrument sinks in water, say to 

 the point \V. If the liquid be heavier than 

 water the instrument will not sink so far; 

 the position of equilibrium in which the 

 weigut of the whole instrument is equal 

 to the weight of the liquid displaced will 

 l>e sooner reached ; and, conversely, if the 

 liquid be lighter than water the instrument 

 \vill -ink farther. Each instrument must 

 lie experimentally graduated by placing it 

 in liquids of known specific densities. By 

 varying the adjustment of the mercury a 

 series of instruments may be made, service- 

 able in ascertaining the specific densities of 

 I'- liquids within particular ranges of density 

 e.g. instruments for sulphuric acid, milk, 

 alcohol, &c. The delicacy of such an instru- 

 ment depends on the bulb C being large and 

 the stem AB thin. The chief modes of 

 I D graduation are (1) Gay-Lussac's areometer 

 or vol umometer. In water the instrument 

 Kg. 1. stands at 100. All the degrees are equal, 

 and each = jjm the volume of that part of 

 the instrument which is immersed when it floats in 

 water. If be the numerical reading when the 

 instrument is floated in a given liquid, the specific 

 rlt-iisity of that liquid is 100 4- n e.g. if the in- 

 trument stand at 80, the specific density = 100/80 

 = 1 '25. (2) Baume, for liquids heavier than water. 

 Water at 17'5 C. = ; an aqueous solution con- 

 taining 10 per cent, by weight of common salt 

 (Xa('l) at 17'5 C. = 10; the scale is uniformly 

 graduated ; specific density = 146 '8 4- (146 '8 - ). 

 (3) Banmc, for liquids lighter than water; 10 per 

 cent, by weight daft-solution at 12'5 C. = ; water 

 at 12-5" C. = 10; specific density = 146 -=- ( 136 '+ 



n). (4) ' Rational ' Baume, for liquids heavier than 

 water; water at 15 C. = ; sulphuric acid, specific 

 density = 1'842 = 66 ; specific density = 144 '3 -=- 

 (144'3-M). (5) Cartier, resembles Baume; for 

 liquids lighter than water, 21 Cartier = 21 Baume ; 

 otherwise 15 Cartier degrees = 16 Baume degrees ; 

 specific density = 136 "8 -=- (126'1 T re). (6) Beck ; 

 pure water = ; specific density, 0'850 = - 30 ; 

 uniform graduation ; specific density = 170 -f (170 

 + n). (7) Twaddell, most used "in England; 

 water = ; graduation not uniform, but readings 

 direct; specific density = (1000 + 5) -=- 1000 e.g. 

 a gallon of acid of 24 Twaddell weighs 10 Ib. x 



10< M00 12<> = 10 lb " x 1-12 = n ' 2 lk < 8 > Tralles, 

 an alcoholometer scale used on the Continent, 

 adjusted so as to show directly the volume- 

 percentage of alcohol in alcohol and water. (9) 

 Sikes, used in the British Customs and Excise ; 

 graduated so as to show how many volumes of 

 water must be added to or taken from 100 volumes 

 of the mixture under examination to reduce it to 

 proof -spirit (a mixture whose density = || that of 

 water at 51 F. i.e. 57O9 Tralles"), the instru- 

 ment being adjustable to different ranges of density 

 by a set of movable weights. Instead of making 

 the quantity of liquid displaced to vary, as in the 

 above instruments, the displacement may be kept 

 constant and the weight of the instrument varied. 

 Fig. 2 shows Nicholson's areometer a hollow 

 brass case, BC ; cups at A and D; a 

 weight at E. Suppose it weighs 2000 

 grains ; and let it sink in water to a 

 certain mark between B and A when 

 500 grains weight is put in A. If it 

 be now transferred to another liquid 

 in which only 250 grains are required 

 to make it sink to the same mark, the 

 second liquid is lighter than water in 

 the ratio of 2250, the whole weight of 

 the apparatus, to 2500, its former whole 

 weight ; and its specific density is there- 

 fore $li$ = 0-9. The same instrument 

 may be used to find the specific density 

 of small solids thus : put a little stone 

 or gem in A ; to make the apparatus 

 sink to the mark say 440 grains are 

 required ; therefore the stone weighs 

 60 grains. Now put it in D. More 

 weights, say 20 grains, must now be 

 put in A ; the 20 grains represent the apparent loss 

 of weight in water ; the specific density = weight 

 in air -f apparent loss in water = 60/20 = 3. By 

 reversing D, which is perforated, the specific den- 

 sity of bodies lighter than water may be ascertained. 

 Fahrenheit's areometer, the original form, differs 

 from Nicholson's in having no platform or cup D. 

 Kousseau's densimeter combines the two methods 

 described above. It bears a cup or cavity at its 

 summit. This is filled successively with various 

 liquids ; each induces a different amount of sink- 

 ing. The instrument-maker has to do the prelim- 

 inary graduation by the use of known liquids. 

 Specific-gravity bulbs are also used ; they are 

 marked with numbers representing specific den- 

 sities. Those which are too heavy sink ; those 

 which are too light float ; the one exactly corre- 

 sponding to the density of the liquid, if there be 

 one, neither rises nor sinks. The most accurate 

 method is that by the specific gravity flask. The 

 specific density of a gas or vapour is determined 

 ( 1 ) by weighing a copper flask when empty, when 

 filled with the gas, and when filled with air, which 

 method gives the density of the gas relatively to 

 that of air, when proper corrections are made so as 

 to compare the two gases at the same temperature 

 and pressure ; (2) by ascertaining the volume occu- 

 pied by a given weight of the gas or vapour at a 



. 



