68 PHYSICAL DETERMINATIONS. 



Determination of Specific Gravity. There are two methods 

 of determining specific gravity, which is, as above stated, the 

 weight of unit volume, or, expressed as an equation 



g = W 



We may determine either the weight of a known volume, or 

 the volume of a known weight. Both methods are used in 

 practice, the first in two ways : 



(1) A vessel of known volume is filled with the liquid and its 

 weight taken. 



(2) A plummet of known volume is immersed completely in 

 the liquid, and the loss of weight due to the displacement of an 

 equal volume of liquid noted. 



The second method is applied (3) by immersing a float of 

 known weight, and noting the volume immersed ; the volume 

 immersed will be equal to a volume of the liquid of weight equal 

 to that of the float. 



Determinations of specific gravity by method (1) are made by 

 specific gravity bottles and Sprengel tubes, by method (2) by 



Fig. 2. Sprengel Tube. 



a Westphal balance, and by method (3) by hydrometers, of which 

 lactometers are special forms of limited range suited for milk. 



For exact determinations of the specific gravity of milk, a 

 Sprengel tube (Fig. 2) presents many advantages. It is a U- 

 shaped tube with narrow capillary ends bent outwards at right 

 angles, one being rather smaller than the other ; the wider of 

 the two has a fine line etched round it, to which the liquid in 

 the tube may be adjusted, the U and the other capillary being 

 completely filled. 



The weight of the dry and empty tube is first ascertained, the 

 tube is then filled with pure distilled water, and immersed in 

 water at exactly 15-55 C. (60 F.) ; when it is seen that no 

 further expansion or contraction takes place, the water should be 

 adjusted to the line on the wider capillary by the cautious appli- 

 cation of a piece of blotting-paper to the end of the narrow 

 capillary ; the tube is then wiped dry and weighed. The differ- 



