TESTING BY THE LACTOMETER 



177 



terant of milk, it was formerly thought that such 

 adulteration could readily be detected by ascertaining 

 the specific gravity; but the results of using the spe- 

 cific gravity may be very misleading, when consid- 

 ered without reference to any other factor. 



INFLUENCE OF TEMPERATURE ON SPECIFIC 

 GRAVITY 



Most liquids expand when heated and contract when 

 cooled. A vessel full of milk or water at 40 F. will 

 overflow when heated considerably higher, that is, 

 will hold less of the fluid, and so the same volume 

 weighs less at higher than at lower temperatures. 

 From this it is readily seen that the specific gravity of 

 a liquid like water or milk grows less when its tem- 

 perature increases. On the other hand, a vessel full 

 of water at 200 F. is not full when cooled to 40 F. 

 The same weight of water occupies less volume and 

 its specific gravity is higher. Decrease of tempera- 

 ture increases the specific gravity of liquids. It is 

 therefore necessary in measuring the specific gravities 

 of different liquids to have the measurements made at 

 the same temperature, if they are to be comparable. 

 The temperature commonly used is 60 F. 



METHOD OF TESTING THE SPECIFIC GRAVITY 

 OF LIQUIDS 



The specific gravity of liquids is readily measured 

 by an instrument known as a hydrometer. The use 

 of such an instrument is based on the fact that, when 

 a solid body floats in a liquid, it displaces an amount 

 of liquid equal in weight to the weight of the floating 



