For a number of reasons a knowledge of the viscosity of igneous magmas is of much importance to the petrologist. In the passage of magma through channels in the Earth, in its pouring out as lava upon the surface, in the sinking of crystals and the rise of gases, and in many other processes in which magma participates viscosity is a dominant controlling factor. Obviously little can be done in the way of direct measurement of the viscosity of magmas while these actions are in progress. Estimates have been made of the viscosity of a lava‐flow in Hawaii from the rate of its flow down a slope (G. F. Becker, Am. J. Sci. , v. 3, p. 29, 1897; his estimate—60 times the viscosity of water—is only about 1/200th of the viscosity of a basaltic melt measured in the laboratory by Kani), and the play of lava‐fountains there and elsewhere has afforded a general idea of the magnitude of the viscosity of basic lavas.