LAW OF FLOATING BODIES 



57 



immersed part of a floating object is equal to the whole weight of the 

 object. If, therefore, the question is asked, how far does an object 

 which floats sink into water the answer is it goes on sinking until 

 it has displaced an amount of water having a weight equal to the whole 

 weight of the floating object. 



Since the depth at which an object floats in water is decided by this 

 rule, we have a ready way of deciding whether an 

 object will sink further in another liquid or not so 

 far. If the liquid into which it is put is less dense 

 than water, like spirits of wine, it is clear that to 

 make up a given weight we shall want more of the 

 liquid. Consequently, to make up a weight equal 

 to the weight of the floating body, the object will 

 have to sink further into the spirit than into the 

 water. If, on the other hand, the object is placed 

 in a liquid such as mercury, which is denser than 

 water, it will not sink so far, because it will not 

 take so much of this denser liquid to have a weight 

 equal to that of the floating body. 



The hydrometer. The construction of a simple 

 instrument called the hydrometer is based upon 

 these conclusions. The instrument is made in 

 various shapes, and graduated differently according 

 to the special use for which it is intended. All 

 hydrometers are, however, used to measure the 

 densities of fluids by observing the depth to which 

 they sink when immersed in them. The lactometer 

 is a form of hydrometer employed for measuring 

 the density of milk. When placed in pure milk a 

 lactometer should float with the mark P (Fig. 45) 

 on a level with the surface of the liquid. In a 

 mixture of milk and water the lactometer floats 

 with some other division level with the surface of 

 the liquid. Thus, in milk 10 per cent, below the average density, the 

 10 above the P mark is level with the surface. 



An experienced observer is, therefore, able from the readings of a 

 lactometer to tell whether a sample of milk has a correct density, or 

 whether it is heavier or lighter than it should be. At the same time 

 it must be clearly understood that it is not possible to decide at once 

 from the reading of a lactometer whether a sample of milk has been 

 adulterated or not. There are other considerations to be taken into 

 account. 



FIG. 45. A hydro- 

 meter for determining 

 the quality of milk. 

 It is called a lacto- 

 meter. 



