Why we don't use hydrometer instead of lactometer?

A hydrometer measures the specific gravity of liquids in general, while a lactometer is specifically designed to measure the density of milk and dairy products. The lactometer is calibrated to account for the unique composition of milk, including fat and solids content, which can significantly affect density readings. Using a hydrometer on milk could lead to inaccurate results, as it wouldn't provide the necessary adjustments for these specific components. Thus, for purposes like assessing milk quality or adulteration, a lactometer is the appropriate tool.