To test the purity of milk, an instrument called a ‘lactometer’ is used. It is a cylindrical vessel made by blowing a heated glass tube. One end of this glass tube is blown into the shape of a bulb and it is filled with mercury, whereas the other end is blown in the shape of a thin tube and is then sealed. It is dipped in pure milk for calibration. ‘M’ is marked at the point up to which it sinks in pure milk. Then the tube is put into water and marked ‘W’ at the point up to which it sinks in water. As milk is denser, the lactometer sinks less in milk as compared to water. The portion between ‘M’ and ‘W’ is divided into three parts, and it marked as 3, 2 and 1 to specify the level of purity.
Now put this instrument in the milk to test its purity. If the instrument sinks up to the mark ‘M’, then the milk is pure. The instrument will sink to a mark higher than ‘M’ if the milk is not pure. The milk is 75% pure if the instrument stands at the mark 3. The milk is 50% pure at the mark 2 and the mark 1 specifies the purity of 25%.
However, the lactometer is not considered as a reliable instrument, as it fails to give the correct assessment of the purity in the case of skimmed milk.