Relative density of a substance is the density of that substance with respect to water.
R . D. = density of liquid / density of water = d_m / d_w
= loss of weight of a solid in milk / loss of weight of that solid in water.
Let us take a clean ball or cube of volume V_s cm^3, density d_s and weighing m grams. Let us take a utensil of milk and then a tub of water. Let us get one spring balance to measure the weight/mass of the ball. Tie a string the ball or cube and attach it to the spring balance. Please find a ball or cube having a density more than milk and water. Like any metal.
1. Measure the mass/weight of the ball in air. Call it W_a.
2. Measure the mass/weight of the ball, dipping the ball completely inside milk. Call it W_m.
3. Measure the mass / weight of the ball by taking it out from the milk vessel and putting it in the tub of water, completely immersing it in water. Call it W_w.
Relative density of milk = d_m = (W_a - W_m) / ( W_a - W_w)
W_a - W_m = [ V_s * d_s * g - (V_s * d_s * g - V_s * d_m * g ) ]
W_a - W_w = [ V_s * d_s * g - (V_s * d_s * g - V_s * d_w * g ) ]
hence the ratio gives d_m / d_w