88 0. JBarus — Isothermals, Isopiestics and 



in which, at the same temperature and pressure, the larger vis- 

 cosities were three or even live times as high as the smaller 

 viscosities, and my work is therefore to this extent independ- 

 ent of the material operated on. Finally marine glue has the 

 advantage of being both adhesive and tenacious, and errors 

 due to slipping (§ 5) are thus reduced as much as possible. 



3. Definitions. — In my paper* on the absolute viscosity of 

 the three states of aggregation, I defined a fluid (liquid or gas) 

 as a body which, under constant conditions of pressure, tem- 

 perature, and stress, shows constant viscosity as to time. In a 

 solid cwteris paribus, viscosity markedly increases \fith the 

 time during which stress is brought to bear. The molecular 

 instabilities of a liquid, therefore, are supplied at the same rate 

 in which they are used in promoting viscous motion. In a 

 solid they are used more rapidly than the small rate of continu- 

 ous supply. 



The j)oint of essential concern in these definitions is the con- 

 stancy of stress, and its value below a certain critical datum. 

 For instance, if in a solid stress be increased at the (small) rate 

 necessary to insure a constant supply of instabilities, then solid 

 viscosity will also be constant, and I am by no means sure that 

 in such a casef yield points would eventually present themselves 

 as breaks in the, continuity of the solid flow. 



On the other hand stress may be conceived to increase so fast, 

 that even a liquid fails to present sufficient instabilities for truly 

 viscous motion. The elasticity and- brittleness of many viscous 

 liquids, especially at low temperatures, is a case in point. 



4. Hardness. — Throughout my work on viscosity,:}: I have 

 adverted to the association of viscosity with zero forces acting 

 for infinite times, and of hardness with infinite forces (relatively) 

 acting for zero times, and have adduced many new examples 

 showing the distinctiveness of these two properties. The 

 subject of hardness has, however, recently taken more definite 

 shape in the researches of Auerbach,§ based on a principle due 

 to Hertz. I According to these observers, hardness is an ex- 

 pression for the elastic limits of a body in case of contact 

 between its plane surface and the curved surface of some other 

 (harder) body. Hardness so denned admits of absolute measure- 

 ment in terms of dynes per square centimeter. 



5. Method of Work. — In all experiments like the present, 

 one cannot be too careful to preconsider the conditions under 



*Phil. Mag., V, vol. xxix, p. 337, 1890. Cf. p. 354. 

 f Of. this Journal, III, vol xxxiv, p. 19, 1887. 



jphiL Mag., V, vol. xxvi, p. 210, 1888. Cf. Bull. U. S. Geolog. Survey, No. 

 73, pp. 42-44, 97, 98. 1891. See § 6 below. 

 §Auerbach, Wied. Ann., vol. xlhi, p. 61, 1891. 

 .[[Hertz, Crelle's Journal, vol. xcvi, p. 156, 1882. 



