70 PHYSICAL DETERMINATIONS. 



" steel-yard " type, carrying a glass plummet at one end ; it is 

 so adjusted that the pointer is at zero when the plummet hangs 

 in air, and is provided with a weight, which, when hooked on to 

 the end, causes the pointer to be at zero when the plummet is 

 immersed in water at 15-55 C. The beam is divided into ten 

 parts, each indicated by a notch, and riders weighing the same, 

 iV T^O* and TuW of tne weight are provided. 



To take the specific gravity, the plummet is immersed in milk 

 at 15-55, and riders are placed on the notches of the beam till 

 the pointer is at zero. If the ^ rider is on notch 3, the specific 

 gravity is 1-03 ; if, in addition, the y^ rider has to be placed on 

 notch 2, the specific gravity is 1-032 ; and if, in addition, the 

 TTrVi) rider has to be placed on notch 4 the specific gravity is 

 1-0324. If a rider is already on a notch, and it is desired to 

 place another thereon, it may be hung on the turned up end of 

 the rider already in position. 



A rule may be given as follows : Count 1 for the weight 

 hung on the end, the first decimal from the notch on which the 

 T rider is hung, the second decimal from the notch on which 

 the ~j rider is hung, the third decimal from the jj^ rider, and 

 the fourth decimal from the y^ 1 ^ rider. 



This method has the advantage of being somewhat more rapid 

 than the use of the Sprengel tube, but is not quite so accurate, 

 as the adjustment of riders and balance cannot practically be 

 performed with very great accuracy. 



In dairy work the lactometer is generally used. From a 

 strictly scientific point of view, there are many objections to 

 lactometers, but their practical convenience is so great that they 

 are instruments of extreme value. 



The faults of lactometers are: (1) They do not indicate 

 true specific gravities, but the inverse of this specific volumes ; 

 consequently, the scale is not divided into equal parts. The 

 divergence from equality is, however, so small in a lactometer, 

 which has only a limited range, as to render it practically 

 admissible to treat the smaller divisions as equal. 



(2) The exact point at which the level of the liquid cuts the 

 stem of the lactometer cannot be ascertained, as, owing to surface 

 energy, the liquid is attracted to a higher level round the stem 

 of the lactometer than the surface of the liquid ; moreover, the 

 height to which the liquid is attracted varies with the nature 

 of the liquid. As milk has always the same composition within 

 narrow limits, there is no practical difference in the height to 

 which it is attracted round the stem ; the eye soon becomes 

 trained in making the proper allowance for this. 



(3) Lactometers are only correct at the temperature at which 

 they are graduated ; at other temperatures their volume varies ; 



