438 



SCIENCE 



[N. S. Vol. XXXVI. No. 927 



currents. In particular, it fails to take into 

 account the relations known to exist between 

 the general permeability of the plasma mem- 

 brane and the demarcation-current potential. 

 Evidence from many sides shows that this po- 

 tential varies with the permeability of the 

 plasma membrane to simple crystalloid sub- 

 stances, decreasing as this permeability in- 

 creases. All cytolytic (i. e., permeability-in- 

 creasing or membranolytic) substances and 

 processes, so far as known, diminish this po- 

 tential. Such facts indicate very clearly that 

 semi-permeable membranes form a funda- 

 mental if not the chief factor in the produc- 

 tion of the demarcation-current potential, and 

 hence also in the production of the action-cur- 

 rent which is evidently due to a variation in 

 this potential. Any sufficient reversible in- 

 crease in permeability would produce under 

 these conditions a negative variation. Ob- 

 viously, numerical data are required for a de- 

 cision of those questions. It is not unlikely 

 that several distinct factors are involved in the 

 production of the action-current, and that the 

 observed eilect is an additive one. The poten- 

 tial of the action-current is said often to ex- 

 ceed that of the demarcation-current, a fact 

 inexplicable on the simple membrane theory. 

 But if an electrical variation due to a chemi- 

 cal change of the kind imagined by Pauli were 

 superposed on one due to altered membrane- 

 permeability, such an effect might conceivably 

 result. Ralph S. Lillie 



Marine Biological Laboratory, 

 September 1, 1912 



SCIENTIFIC BOOKS 

 PhysiJc in grapMschen Darstellungen. Von 



Felix Auerbach. Leipzig, B. G. Teubner. 



1912. Large 8vo. 



In recent years Professor Auerbach has been 

 devoting himself, with success, to encyclo- 

 pedic treatises on physics. It is not so long 

 since he published his excellent " Kanon der 

 Physik." But the present book is decidedly 

 more novel in design. It will, in particular, 

 be invaluable to teachers. It lends itself at 

 once to the construction of lantern slides for 

 the graphic illustration of involved points in 



theory. It is furthermore an aid to the set- 

 ting of graphic problems in all parts of the 

 subject. In a lecture course on light, for 

 instance, almost all the answers to questions 

 can be given by graphs. Such an exercise is 

 easily corrected on the one hand, while on the 

 other it is exceedingly difficult for the student 

 to answer the question by mere copying. It 

 has therefore always seemed to the writer that 

 a similar body of questions, carried through- 

 out the whole of physics, all of them to be an- 

 swered graphically, would meet many of the 

 difficulties now encountered in case of a lec- 

 ture course. It is probable that Auerbach's 

 book is a definite contribution in this direc- 

 tion and that a systematic course of questions, 

 to be answered by drawing, may be put to- 

 gether by means of it. 



Among the great variety of diagrams and 

 constructions given, all of about the same 

 importance, it will only be possible to refer to 

 a few at random. Thus the curious repre- 

 sentation of dimensional formulse obtained by 

 laying off the powers of c, g, s, in terms of 

 length, breadth and thickness, is new to the 

 writer. Graphic classification of different 

 orders of standard magnitude in physics, as, 

 for instance, the prominent distances, times, 

 velocities, densities, etc., occurring in me- 

 chanics and the vast number of data in other 

 parts of physics, are bound to be convenient 

 for reference. Constructions relating to equi- 

 potential surfaces and lines of force are given 

 in familiar diagrams, but the plates contain 

 suggestive cases of graphic statics, including 

 standard trusses. Similarly the velocity and 

 acceleration hodographs adduced are cleverly 

 chosen. The representation of the cylindroid, 

 however, seems to the writer inadequate. 



The subject of elastics both on its experi- 

 mental and theoretical side lends itself ad- 

 mirably to graphic treatment, and a great 

 variety of constructions is given, including 

 impact, viscosity, hardness, etc. In hydro- 

 dynamics the plates abound in practical appli- 

 cations of the subject, in addition to the many 

 exhibits of flow for cases of both rotational 

 and irrotational motion. Waves are particu- 

 larly well illustrated and the final develop- 



