A demonstration of osmotic pressure exerted by fat

Abstract

In the first of two demonstrations, the authors lowered a cylindrical rubber bag, one and one half inches in diameter and eight inches long, into an oiled muslin bag of about the same dimensions. The rubber bag was then filled to overflowing with olive oil. The rubber bag expanded, as the oil filled it, to the full length and width of the muslin sheath. The sheath prevented further extension of the rubber bag and imparted rigidity to the osmometer that was ultimately constructed. The full double bag, with its mouth wide open, was then raised so as to enclose about an inch of the lower end of a long glass tube which was firmly supported vertically above the demonstration table. The glass tube was 5 feet long and its bore was 4 mm. in diameter. Ligatures were tightly secured around the neck of the double bag against the immersed lower end of the vertical tube. The bag then hung directly from the end of the tube. The bag and its sheath were in a tightly distended condition and a stationary column of oil an inch high in the tube was visible above the protruding edge of the sheath. The tube and bag were then lowered into a salt mouth liter bottle on the table until the bag almost touched the bottom of the bottle. The height of the bottle and the length of the bag were nearly equal. The tube was then marked with a label on the plane of the oil meniscus just above the neck of the bag, and enough ether was poured into the bottle to provide immersion for the bag to the depth of an inch. For a moment no change in the volume of oil was apparent, and the lateral pressure of the ether was obviously without mechanical effect. But in a minute or two diffusion currents were visible along the surface of the bag and oil rose rapidly in the tube.