538 



NATURE 



[October 5, 189: 



sence of cilia, and the existence of a pseudppodic extru- 

 sion of hyaline protoplasm, are carefully given. The 

 author wisely inclines to the last. It is certain that one 

 of the results of the use of apochromatic objectives during 

 the last three or four years has been to enable us to de- 

 monstrate that not only are there perforations in the sili- 

 ceous tests of the diatoms, but that in the raphe of some 

 Navicular and kindred forms, there is a ''great" per- 

 foration, which runs tube-like from the apices of the 

 frustule to the central nodule ; and this may be readily 

 seen to lend itself to the pseudopodic extrusion and 

 withdrawal of protoplasm ; and we commend the study of 

 the possibility of this to microscopists. Delicate stains 

 may be used that will not immediately destroy the or- 

 ganism, and that will tend to make the " hyaline proto- 

 plasm ' at least more manifest. But in this connection 

 the work of Biitschli and Lauterborn cannot be neglected. 

 Making Pinnitlaria nobilis the subject of research, they 

 specially directed attention to its mode of motion. The 

 motion in diatoms is of a peculiar kind, being frequently 

 a series of jerks which carry forward the frustule in the 

 direction of its length, and often carry it back along the 

 same path. Yet the motion may be smooth and 

 equable. 



Biitschli conceived the idea of placing under the thin 

 covering glass, laid upon the top of the water in which 

 he was microscopically studying the Pinnitlaria, a minute 

 drop of Indian ink. This in its ultimate particles is, of 

 course, not soluble. Its extremely fine granulation was 

 therefore of great value, for by means of the enormous 

 multitude of these black granules he affirms that he was 

 able to see an extremely fine thread, which was directed 

 backwards. This, he contends, was a protoplasmic 

 filament, but so fine, and, as we apprehend, so near in 

 its refractive index to that of water, that it is otherwise 

 invisible. 



This filament, it is stated, is formed by jerks, and 

 the diatom was simultaneously moved in the opposite 

 direction ; while at times the filament appears to be 

 retracted. 



That these results are of value, there can be no doubt, 

 and they open a line of study that may be most profit- 

 able. 



Mr. Mills has adopted the method of classification for 

 the Diatomaceffi which for the present may fairly be 

 considered the best ; but we can but fervently hope that 

 a series of detailed discoveries will at no very distant 

 date make such generalisation possible as will super- 

 induce a great simplification in this direction. 



There is a very useful chapter on Mounting Diatoms, 

 and some excellent teaching on the microscopic ex- 

 amination of these forms ; and the whole is rendered 

 complete by a chapter that will greatly aid the beginner, 

 on " How to Photograph Diatoms." 



We welcome this book ; it will occupy a distinct place 

 in the literature of the subject in our language at present, 

 and will, we hope, make the way for a greatly enlarged 

 and amplified second edition. There is much to praise 

 in the volume, and what we have endeavoured to point 

 out as deficiencies we do not treat as defects. The sub- 

 ject is so large that an author may well pause and wonder 

 at what point an " Introduction" to such a subject should 

 NO. 1249, VOL. 48] 



halt in details. But we think that what has been given 

 will open the way for very much more, and hope that 

 Mr. Mills may be called upon and induced to provide it. 

 We note some printer's errors in the book. It will 

 suffice to call attention to page 6, where a period at the 

 end of the second line destroys the sense ; to the word 

 ' rhizopodo " for " rhizopodia '' on page 1 3 ; to the wrong 

 spelling of an author's name, as in the foot-note on page 5, 

 and to a reference to " northern microscopic " for 

 "northern microscopist" on page 159. D. 



THE PROPAGATION OF ELECTRIC ENERGY. 



Untersuchiinge/i iiber die Ausbreitung der Electrischen 

 Kraft. Von Dr. Heinrich Hertz. Pp. 295. (Leipzig ; 

 Johann Barth.) 



A DISCOVERER'S own account of his work is 

 ■^^ always of interest, and when it is an epoch- 

 making work and the account so clear and well described 

 as to be intelligible to all, it deserves the most careful 

 attention, and should be studied by all who fe;l any 

 interest in the subject. Dr. Hertz's account of his dis- 

 covery of the propagation of electric energy is eminently 

 a work of this kind. The subject is of immense import- 

 ance ; the work described is of the highest order of ex- 

 perimental investigation ; the results attained have 

 contributed more than any other recent results to 

 revolutionise the view taken by the majority of scientific 

 workers as to the nature of electromagnetic actions. It 

 is to be hoped that a translation of this account of one of 

 the greatest advances in our knowledge of nature will 

 soon be in the hands of all who care to learn how the 

 functions of the ether have been raised from obscurity 

 into light, from being in the opinion of many fi pious 

 belief to be the momentous question of the hour. Prot 

 Hertz gives in his introduction an interesting account of 

 the steps by which Maxwell's theory may be connected 

 with the older theories. These latter supposed action at 

 a distance pure and simple, and postulated two fluids, &c., 

 &c. They neglected the intervening medium. The second 

 step was to introduce the medium as performing some 

 function when it was a material medium, but still to 

 retain the positive and negative electricities acting across 

 the space from molecule to molecule. This was practi- 

 cally Mossotti's theory as to the properties of the dielectric 

 founded on Poisson's theory of magnetic induction. M. 

 Poincard seems to have got to about this stage, or per 

 haps a little further. The third stage was to transfer th£ 

 molecular action to the ether, but still to consider it a; 

 due to electrical fluids attracting and repelling one 

 another, producing the etherial stresses. The fourth 

 stage was to see that these attractions and repulsions o 

 electrical fluids are quite superfluous, and to attributf 

 the whole phenomenon to stresses in the ether set up bj 

 straining it. In this last stage there is no room for ar 

 electrical fluid with attracting and repelling properties 

 and accordingly it is suppressed. What the structure o 

 the ether may be which is strained, and thereby electro 

 magnetic stresses produced, is still unknown, and conse 

 quently the nature of the strain is unknown. It certainlj 

 differs from the ordinary straining of a solid in two im 



