Thixotropy (thixein, to smear; tropein, to change) has recently been applied to a phenomenon of gels and sols which really has been known for some time, but whose significance has merely begun to dawn. Some systems will decrease in viscosity when stirred. Such are certain gelatine gels, rubber cements, and certain others as Portland cement. In case of the glycerine and litharge mixtures whose thixotropic properties I am reporting elsewhere, a change from a solid to a very plastic state may be brought about by violent mechanical agitation. These changes in viscosity, hardness, plasticity, and solidity with mechanical agitation are connected with thixotropy. A liquid or a sol whose rate of flow through small tubes is not proportional to the pressure gradient may be said to be thixotropic also. There are many illustrations of this in albumins, celluloses, and suspensions, oils and greases. The phenomenon of thixotropy gives important clues to the nature of the structure of these colloid systems. Such changes immediately suggest analogies to the sensibility of living biological systems to mechanical stimulation. Also after a thixotropic system has been agitated the transformation from the less viscous to the more viscous state has analogy with recovery in living organic systems.