The test is based on the observation that a suitable amount of sulfuric acid added to the milk will dissolve proteins and other components, except the fat. Heating and centrifuging cause the fat to separate and float to the top, in a layer free of bubbles. The amount of fat in the milk can then be estimated from the volume of that layer. The procedure was commonly carried out in a special flask with a long neck, called a Babcock bottle.
