5 i2 THE POPULAR SCIENCE MONTHLY. 



and is producing immense changes. And in the future it is destined 

 still more profoundly to alter our mechanical ideas and activities : 

 the great revolution there is only just beginning ; another half-cen- 

 tury is yet needed fully to develop it. 



These two great principles evolution and the conservation of en- 

 ergy form the main bulk of our age's addition to the world's accu- 

 mulated stock of knowledge. But among the separate sciences many 

 wonderful advances have also been made which can not be overlooked 

 in the briefest retrospect of the half-century's gains. To these a few 

 words must next be devoted. 



Among sciences of the abstract-concrete group, electricity had 

 hardly got beyond the stage of an elegant amusement at the opening 

 of our epoch. Statical electricity was still the department about 

 which most was known. Galvanism as yet stood apart as a distinct 

 study. Its connection with magnetism had not long been proved by 

 the discoveries of Oersted. In 1837 itself, however, Wheatstone 

 constructed the first telegraph. From that moment, under the foster- 

 ing care of Faraday, Daniell, Cooke, Morse, Arago, Tyndall, Edison, 

 and Thomson, electric science became a power in the world. The 

 whole theory of electricity as a mode of energy has since been fully 

 explored and expounded. A vast field has been added to science. 

 Units and modes of absolute measurement have been invented. The 

 telephone and microphone have been introduced ; secondary batteries 

 have been formed and improved ; the dynamo has become a common 

 object of the country ; and the electric light has grown under our very 

 eyes into a practical and extremely dazzling reality. Electricity, as 

 we know it, with all its manifold useful applications, is almost entirely 

 a creation of the last half-century. 



In physics the present epoch, though chiefly remarkable for the 

 series of investigations which led up to the discovery of the law of 

 conservation, has also illustrated many minor principles of the first 

 importance. The true theory of heat and the laws of radiant energy 

 have been definitely formulated. The undulatory theory of light a 

 discovery of the previous quarter-century has been universally 

 adopted and justified. Thermo-dynamics have been elevated into a 

 great and increasing branch of science. Sir William Thomson's law 

 of dissipation of energy has completed and rounded off the theory of 

 conservation. The causes and methods of glacier-motion have been 

 investigated and established. Photography has almost passed through 

 its entire life-cycle. The polarization of light has been observed and 

 studied. Spectrum analysis has come into the front rank as an instru- 

 ment of research. In short, a greater number of new physical phe- 

 nomena have been discovered or old ones interpreted than in the whole 

 space of previous time put together. 



In chemistry, the advance has been more in detail than elsewhere. 

 Chemical science alone still remains a somewhat fragmentary mass 



