592 



SCIENCE. 



[N. S. Vol. XXIV. No. 619. 



delayed germination of seeds, discovering that 

 the cause generally lies in the seed coats, 

 rather than in the embryos. In the case of 

 the well-known ' upper seed ' of the cocklebur, 

 it is found that the delay is secui-ed by the 

 seed coat excluding oxygen; while in many 

 seeds it is secured by the coats excluding 

 water. John Donnell Smith publishes his 

 twenty-eighth paper entitled ' Undescribed 

 Plants from Guatemala and Other Central 

 American Kepublics.' Clayton O. Smith de- 

 scribes with the aid of half-tone illustrations 

 a bacterial disease of Oleander occurring on 

 young Oleanders in California, which proves 

 to be the same disease that causes knot-like 

 growths on the olive. 



The November Journal of Nervous and 

 Mental Disease opens with a discussion of the 

 cerebral element in the reflexes and its relation 

 to the spinal element by Dr. G. L. Walton 

 and Dr. W. E. Paul. Dr. Wharton Sinkler 

 contributes the report of a case of Landiy's 

 paralysis with the unusual result of recovery, 

 and Dr. Morton Prince gives an account of a 

 case of brain tumor characterized by a cir- 

 cumscribed limited area of anesthesia, marked 

 muscular atrophy appearing early, epileptiform 

 attacks of hemialgesia of a peculiar nature; 

 loss of the muscular sense, astereognosis, 

 ataxis and paresis, increasing to ultimate 

 paralysis. A study by Dr. Harvey Gushing 

 of sexual infantilism with optic atrophy in 

 cases of tumor affecting the hypophysis cerebri 

 completes this part of the number. 



DISCUSSION AND CORRESPONDENCE. 



I A DEFINITION OF FLUID. 



In discussing Professor Elihu Thomson's 

 paper on vulcanism Dr. Alfred C. Lane^ asks : 

 ■*' Are we clear as to where we are to draw the 

 line between viscous fluid and solid ? " and 

 he then gives a definition of fluid which is 

 certainly an improvement on the old descrip- 

 tion of a fluid as a substance which will con- 

 form to the shape of the containing vessel and 

 will present a level upper surface. 



Dr. Lane's definition, however, does not 

 answer the question: Where shall the line 



^Science, N. S., Vol. XXIV., No. 613, p. 404. 



between solid and fluid be drawn? He cites 

 molasses candy and tar as bodies which ap- 

 parently are solids but which really are fluids ; 

 but he takes no account of the fact that it 

 is changing conditions external to matter 

 which determine the appearance and disap- 

 pearance of many of its physical and chemical 

 properties. At the temperature of liquid air 

 both molasses candy and tar would lose the 

 property of highly viscous fluidity, which they 

 assume under the ordinary conditions where- 

 under we have become familiar with them, 

 and would acquire a rigidity which would 

 make them solids in the most approved sense. 

 The place to draw the line between solid and 

 fluid is at the junction of conditions which 

 determine the supercession of the solid by the 

 fluid state. This point was long ago recog- 

 nized as a physical constant of matter and 

 has received a name — aplastic yield-point. 

 This point, like melting-point, boiling-point 

 and simple gas-point, separates two of the 

 phases or states of aggregation through which 

 matter passes in the course of its transforma- 

 tion from the highest to the lowest degree of 

 polymerization. 



Plastic yield-point, like melting-point and 

 boiling-point, is a function of both tempera- 

 ture and pressure, and can, therefore, be rep- 

 resented graphically by a curve drawn with 

 reference to temperature and pressure as rect- 

 angular coordinates. Under conditions fixed 

 by a point on the curve of its plastic yield- 

 point, a substance is on the dividing line be- 

 tween its solid and fluid phases. An increase 

 of either temperature or pressure would (if a 

 path be open) occasion molecular shearing — 

 that is, differential motion between molecules 

 — at any point in the mass. A decrease of 

 either temperature or pressure would place the 

 substance under conditions represented by the 

 area on the other side of the curve, where the 

 intermolecular locking is strong enough sen- 

 sibly to resist (within the time considered) 

 the force tending to make the molecules glide 

 over one another; this is the distinctive prop- 

 erty of solids. For all purposes of mechanics, 

 including cases arising in dynamic geology, a 

 fluid may be described as matter exposed to 

 conditions which place it in the region beyond 



