A DAIRY LABORATORY GUIDE 2/ 



THE SPECIFIC GRAVITY OF MILK AND 

 THE LACTOMETER 



The specific gravity or density of a substance is 

 a ratio between the weight of a given volume of 

 the substance and the weight of a given volume of 

 some other substance taken as a standard. The 

 standard for liquids is water and its specific gravity 

 is taken as one. Milk is slightly heavier than water, 

 and the specific gravity of normal milk averages 

 1.032, and the specific varies from 1.029 to 1.035. 

 If we add water to milk the specific gravity is 

 lowered, because water is lighter than milk; and if 

 we skim milk we take away fat which is lighter 

 than milk and the specific gravity is increased. 

 When the water is added to milk the effect is the 

 same as it would be if the solids were actually re- 

 moved from the milk. This is true because the 

 solids which were distributed throughout a given 

 volume on the addition of water, have to 

 distribute themselves throughout the increased 

 volume of the liquid. For this reason we say 

 that the addition of water to milk " decreases " 

 the total solids, and it is a fact that when water is 

 added to milk all of the solids are decreased in the 

 same proportion. It is important to remember 

 this fact when one is figuring out the per cent of 

 adulteration of milk. 



The Lactometer. — The instrument used to 

 measure the specific gravity of a liquid is called a 

 hydrometer, and there are many kinds of specialized 

 hydrometers. The hydrometer used to test the 

 density of milk is called a lactometer, and, for the 

 most part, only two kinds are used. One of these 

 is called the Quevenne (called Q. for abbreviation) 

 and the other is called the New York State Board of 



