272 



THE POPULAR SCIENCE MONTHLY. 



the other books, and will, therefore, not 

 be a favorite with teachers in schools, 

 and can hardly bo expected to have the 

 remarkable success of the other Primers. 



LITERARY NOTICES. 



Chemical Exercises in Qualitative Anal- 

 ysis. For Ordinary Schools. By George 

 W. Rains, M. D., Professor of Chemistry 

 in the University of Georgia, New York : 

 D. Appleton & Co. Pp. 59. Price, 50 

 cents. 



Under this modest title, and within very 

 moderate limits, Professor Rains has made 

 a very considerable contribution to sound 

 scientific education. He has had much ex- 

 perience in introducing boys into chemistry, 

 and the course of exercises here worked out 

 he has long verified in practice. His object 

 is to bring the minds of pupils into imme- 

 diate contact with Nature, and so he puts 

 them at work, at the outset, to find out by 

 trial the chemical properties of substances. 

 His little book provides for no recitations, 

 but for elementary chemical work. The 

 learner is not told ; he finds out the proper- 

 ties and reactions of bodies by testing them 

 and by experiment. His progress consists 

 in solving problems, and making what are to 

 him a course of new discoveries. The book 

 is based upon the idea that mere book- 

 knowledge in chemistry is a sham and an 

 imposture. 



To facilitate the mode of study adopted, 

 Professor Rains has devised an ingenious 

 and most convenient portable laboratory, to 

 which his manual is adapted, and which will 

 be a great help to students, whether work- 

 ing alone or in school-classes under a teacher. 

 We will give a drawing next month of this 

 useful contrivance, and describe Professor 

 Rains's method more fully. 



Henry's Contribution to the Electro- 

 Magnetic Telegraph. With an Account 

 of the Origin and Development of Pro- 

 fessor Morse's Invention. By William 

 B. Taylor. Reprinted from the Smith- 

 sonian Report for 1878. Washington : 

 Government Printing-Office. 1879. 



The name of Professor Henry is not 

 among those who are associated in the popu- 

 lar mind with the electric telegraph, and yet 



without his discoveries the electro-magnetic 

 telegraph of to-day could not exist. Though, 

 in making his electric investigations, he 

 was not working with the aim to construct 

 a telegraph, he yet clearly perceived the 

 bearing of his results upon such a system 

 of communication. The telegraph has been 

 a growth to which many minds contributed, 

 and it is desirable that the labors of each 

 of the contributors should be placed in such 

 relations as to show their comparative value. 

 This Professor Taylor has done in the above 

 pamphlet, in which the remarkable investi- 

 gations of Professor Henry receive a recog- 

 nition that their importance deserves. Pro- 

 fessor Taylor reviews the attempts to operate 

 a telegraph by f rictional electricity, then by 

 galvanism, by galvano-magnetism, and final- 

 ly by means of the electro-magnet. Professor 

 Taylor thus states the contribution of Pro- 

 fessor Henry to the solution of the problem : 

 He has, he says, " the preeminent claim to 

 popular gratitude of having first practically 

 worked out the differing functions of two 

 entirely different kinds of electro-magnet: 

 the one surrounded with numerous coils 

 of no great length, designated by him the 

 'quantity' magnet, the other surrounded 

 with a continuous coil of very great length, 

 designated by him the ' intensity ' magnet. 

 The former and more powerful system, least 

 affected by an ' intensity ' battery of many 

 pairs, was shown to be most responsive to 

 a single galvanic element: the latter and 

 feebler system, least influenced by a single 

 pair, was shown to be most excited by a 

 battery of numerous elements; but at the 

 same time was shown to have the singular 

 capability (never before suspected nor im- 

 agined) of subtile excitation from a distant 

 source. Here for the first time is experi- 

 mentally established the important principle 

 that there must be a proportion between 

 the aggregate internal resistance of the bat- 

 tery and the whole external resistance of 

 the conjunctive wire or conducting circuit; 

 with the very important practical conse- 

 quence that, by combining with an ' intensk 

 ty ' magnet of a single extended fine coil an 

 ' intensity ' battery of many small pairs, its 

 electro-motive force enables a very long con- 

 ductor to be employed without sensible dim- 

 inution of the effect." These investigations 

 of Henry were made from 1829 to 1831. 

 They made the magnetic telegraph, which 



