152 Scientific Intelligence. 



The sign of the effect is positive in cobalt, iron, steel, antimony, 

 tellurium, lead, zinc, cadmium and negative in the other metals. 

 — Nature, p. 185, June, 23, 188V. j. t. 



V. Practical Electricity ; a Laboratory and Lecture Course for 

 first year Students of Electrical Engineering, based on the practi- 

 cal definitions of the Electrical Units. By W. E. Ayrton, 

 F.R.S., etc. 16mo, pp. xvi, 516. London, 18S7. Cassell & Com- 

 pany. — This book is intended, as the preface states, for first year 

 students in electrical technology at the City and Guilds of Lon- 

 don Central Institution at South Kensington. In arranging the 

 order of subjects, the author has followed what appears to him to 

 be " the natural as distinguished from the scholastic method of 

 studying electricity." The first subject considered is current, 

 then follows potential difference, and lastly resistance. The con- 

 cluding chapters are devoted respectively to a discussion of 

 Current generators, of Insulation, of Quantity and Capacity, of 

 Commercial Ammeters and Voltmeters, and of Power and its 

 measurement. The treatment of these subjects seems to be 

 admirable. The book assumes only a moderate amount of pre- 

 vious training, and hence states what the student should know in 

 quite an elementary way, but clearly, concisely, and logically. 

 In speaking of current, for example, it says : " In reality we are 

 sure neither of the direction of flow of an electric current nor 

 whether there is any motion of anything at all ;" and hence, " the 

 statement that an electric current is flowing along a wire is only 

 a short way of expressing the fact that the wire and the space 

 around the wire are in a different state from that in which they 

 are when no electric current is said to be flowing." The use of 

 the letters P. D., potential difference, in place of the old E. M. F. 

 is an important step of progress, and one which therefore should 

 be generally adopted. All the values given are based upon the 

 units adopted by the Paris Electrical Congress of 1884, though 

 as they have not yet been legalized in this country or Great 

 Britain, it is not easy to see why they are called " legal " units. 

 The apparatus described is largely original and is well illustrated. 

 A good feature which the author has adopted is to mount perma- 

 nently on the same board the complete apparatus required for each 

 experiment. Excellent examples by way of illustration are fre- 

 quently introduced and the book closes with an appendix which 

 considers Kirch h off' s laws of divided circuits and which gives 

 several specimens of the instructions given to the students for 

 performing the experiments. Professor Ayrton is well known as 

 an able and indefatigable worker in the field of electrical science, 

 pure as well as applied, and we think that the book before us 

 will add to his reputation as being for its clearness, scientific 

 accuracy, originality, and admirable method of arrangement the 

 best book which has yet appeared on the subject of which it 

 treats. g. f. b. 



8. On the Synthesis of Juglon. — Several years ago yellow needle- 

 shaped crystals were observed on the outer coating of walnuts 

 gathered towards the close of June. These were examined by 



