262 



NATURE 



[Feb. 3, 1876 



of the ions," &c. Some of the illustrative analogies given 

 by the author are "also very happy, as, for example, the 

 ease with which the molecular transfer is effected in elec- 

 trolysis is compared to the ease with which a chain hang- 

 ing over a pulley is moved : " When the two sides are 

 equal each link on one side may be conceived as keeping 

 in equilibrium the opposite link on the other side. A slight 

 force pulling one side down will bring each link opposite 

 to a different one" (p. 137). Incidentally, one or two of 

 the new things strike us as open to question : for instance, 

 the habitual use of the word isolate instead of insulate ; 

 the former has a French aspect, and certainly is less 

 familiar to English readers than the latter term. Again, 



X t + 



<■ "X HT ^ ' 



+ 



Fig. I. 



the omission of all names of discoverers, because, the 

 author states in the preface, " the book is not a history of 

 discovery ; " nevertheless, is it not well that students 

 should be able to associate with Faraday's name, for in- 

 stance, the famous discoveries he gave to the world .? and 

 with all the author's care one or two less important names 

 have crept in, that thus have an undue prominence given 

 to them. On the other hand the unostentatious tone of 

 the book and the entire omission of any reference to the 

 writer, even in the description of the instruments he has 

 devised or the facts he has discovered, are excellent traits, 

 and quite characteristic of the author. 



i \ It 



through a considerable range of temperature. At a lower 

 temperature yet, but still at a dull red heat, the ball begins 

 to be able to receive -\- electricity, and shortly after, as it 

 cools, it accepts both kinds with nearly equal readiness. 

 . . . Again, if we take two equally-charged gold-leaf elec- 

 troscopes (Fig. i), one charged with -\- and the other with 



- electricity, and if we bring earth-connected white-hot 

 iron balls a few inches above the caps of each, they will 

 be discharged with nearly the same facility. On repeating 

 the experiment continually, as the balls cool, it is found 

 that A, or the +ly charged electroscope, ceases to be dis- 

 charged, though the ball is of a red heat, while B, or the 



— electroscope, continues to be immediately discharged, 

 although the ball has lost all incandescence." 



The explanation of this phenomenon Dr. Guthrie takes 

 to be as follows :—" If we conceive the air-bathed and 

 electrically air-straining masses of iron, A and B, to be '\ 

 respectively +ly and — ly electrified, and then to be gra- 



Fig. 2. 



The following extracts will illustrate the remarks we 

 have made, and afford our readers an idea of the experi- 

 mental portion of this treatise. Here is an experiment of 

 the author's which is of considerable interest, whatever 

 explanation may be accepted : — 



" When at a bright white heat, an iron ball refuses to 

 receive, or at least to retain, even for a moment, a charge 

 of either -}- or — electricity. On cooling down, but while 

 still of a red heat, it acquires the power of receiving a — 

 but not a + charge ; and this distinction is maintained 



Fig. 



dually heated, the air which we have already partly sc. 

 to have a greater attachment to -f- than to - electrici; 

 will, supposing the attachment between the metal ai 

 both kinds to be equally diminished, succeed first, in tl 

 molecular turmoil at the heated surface, in carrying av ; 

 the -f " (p. 81). Hence the apparently opposite effect sc, 

 in Fig. I is due to the inductive action of the charged ek 

 troscope upon the earth-connected ball over-head, so th 

 the ball in A is negatively electrified and retains its char- 

 whereas in B the ball becomes positively electrified ar 

 dissipates its charge as fast as it is renewed, and hen- 

 in this case the electroscope is discharged as if by a poii 

 The same power of discharge can be shown by replacii 

 a hot ball with a platinum wire made white hot by a ci 

 rent. Fig. 2 shows an electroscope and a Leyden .i 

 being discharged in this manner. 



