When a fluid traverses a straight tube of sufficient length the individual molecular constituents arrange themselves eventually parallel to the long axis of the tube. Moreover, the cylinder of fluid does not move forward as one unit, but in such a manner that the central portion attains the greatest speed of flow, while the more external layers show a decreasing velocity from within outward. Thus, the layer of fluid which is in contact with the vessel-wall will show no movement whatever, provided that it moistens the wall. It must be clear, therefore, that the individual particles composing a fluid must rub on one another. Elements in neighboring layers move at different speeds and while some molecules are separated from one another, new contacts are constantly formed. The movement of the fluid is therefore accompanied by an internal, or molecular friction. If different fluids of the same temperature are forced through