190 



SCIENCE 



[N. S. Vol. XXV. No. 631 



affirmative; consequently, it should be demon- 

 strated that my reasons are no good. Nobody 

 ever attempted this, and when arguments were 

 given purporting to be opposed to my ideas, 

 these invariably were not my views but only 

 what the critic fancied to be my views." 



A. E. Ortmann 

 Cabnegie Museum, Pittsbukg, Pa., 

 October 4, 1906 



SPEOfflO NAME OF NEOTURUS MAOULOSUS 



In the last number of the American Nat- 

 uralist (Vol. XLI., January, 1907, pp. 23-30) 

 there is an elaborate paper by Professor F. C. 

 Waite under the above title, in which he shows 

 that the name employed there has the priority 

 over N. maculatus, the term most commonly 

 adopted by anatomists. Towards the end of 

 the paper (p. 27) he makes the following state- 

 ment : " In the past ten years although many 

 papers have been written on Nectwrus, two 

 only have, as far as I know, used the correct 

 nomenclature." 



I wish to say that the ' correct ' name was 

 pointed out and the proper references given 

 by the late Dr. G. Baur as early as 1897 (Zool. 

 Bull., I., p. 41). Since then it has been em- 

 ployed by various systematists. Thus the 

 name N. maculosus is used in the eighth edi- 

 tion of D. S. Jordan's ' Manual of the Verte- 

 brate Animals of the Northern United States,' 

 1899, p. 175, in which I tried to bring the 

 nomenclature up to date. It has since been 

 used, both in this journal (Science, N. S., 

 XI., 1900, p. 555) by Fowler, and in the Amer- 

 ican Naturalist (XL., 1906, p. 159) by Stone. 

 Leonhard Stejneger 



Smithsonian Institution, 

 January 14, 1907 



THE DEFINITION OF SOLID AND FLUID 



To THE Editor of Science: — The point I 

 have raised (October 26) as to the definition 

 of solid and fluid seems quite timely in view 

 of the discussion going on between Hoskins 

 and See, and the letter of Mr. Willcox (No- 

 vember 9). Note the use of the term 'solid' 

 in one, of ' substance ' in the other, of the 



" See also my reply to Gager's criticism in 

 Science, August 17, 1906, pp. 214-217. 



two deiinitions of rigidity cited by Hoskins. 

 Their difference seems to be as to whether it 

 is proper to speak of the rigidity of a fluid 

 or a gas. The real question of fact, how 

 much the interior of the earth yields to a cer- 

 tain variation of pressure, has not thus far 

 entered the discussion. 



Again, Mr. Willcox defines fluid and solid 

 quite other than was suggested by me and the 

 line between as the curve of the plastic yield 

 point. 



His definition is quite tenable, if we agree 

 to it, may be made as exact, and fits quite as 

 well the Latin derivation of the word fluid, 

 but I am not sure that it agrees as well with 

 usage or is as practical. We could then speak 

 of no substance as solid or fluid without 

 knowing under what pressure it is. Whether 

 a body were solid or fluid would then depend 

 not merely on the state of the body itself, 

 including its temperature, but also on its sur- 

 roundings — the pressure. We cannot, then, 

 as he writes me, ' properly refer to any sub- 

 stance as a plastic solid.' 



The earth's interior would be classed as a 

 fluid, and not, as has been lately common, on 

 account of its high rigidity, as solid. 



The one point which is not quite clear, as 

 he brings it in parenthetically, is whether the 

 plastic yield point, and so his definition, de- 

 pends on the time or rate of application of 

 pressure. I judge not, according to the molec- 

 ular theory which he adopts (dear to T. 

 S terry Hunt) that there are three states of 

 molecular aggregation, solid, fluid and gas, 

 and that the solid molecules are heavy and 

 complex aggregates of the liquid molecules, as 

 these are in their turn of the gas, and that 

 sufficient temperature and pressure will break 

 up the large solid molecules. 



The definition which occurred to me, that a 

 fluid is a body that can not rest under stress, 

 i. e., in a strained condition, is, however, just 

 as definite and draws just as sharp line as that 

 of Mr. Willcox. We may express it in hia 

 terms thus — a -fluid has a temperature such 

 that Us plastic yield -point is reached even at 

 zero pressure. The relative content of the 

 two concepts can be expressed graphically 

 thus. 



