Page:Popular Science Monthly Volume 17.djvu/284

272 the other books, and will, therefore, not be a favorite with teachers in schools, and can hardly be expected to have the remarkable success of the other Primers.

 

this modest title, and within very moderate limits, Professor Rains has made a very considerable contribution to sound scientific education. He has had much experience 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 immediate 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 properties 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 working 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.

name of Professor Henry is not among those who are associated in the popular 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 investigations of Professor Henry receive a recognition that their importance deserves. Professor Taylor reviews the attempts to operate a telegraph by frictional electricity, then by galvanism, by galvano-magnetism, and finally by means of the electro-magnet. Professor Taylor thus states the contribution of Professor Henry to the solution of the problem: He has, he says, "the preëminent 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 imagined) of subtile excitation from a distant source. Here for the first time is experimentally established the important principle that there must be a proportion between the aggregate internal resistance of the battery and the whole external resistance of the conjunctive wire or conducting circuit; with the very important practical consequence that, by combining with an 'intensity' magnet of a single extended fine coil an 'intensity' battery of many small pairs, its electro-motive force enables a very long conductor to be employed without sensible diminution of the effect." These investigations of Henry were made from 1829 to 1831. They made the magnetic telegraph, which 