ADDEESS. IXXV 



Dr. Kerr on the dielectric state, from -which it appears that when electricity 

 of high tension is passed through dielectrics, a change of molecular arrange- 

 ment occurs, slowly in the case of solids, quickly in the case of li(]uids, and 

 that the lines of electric force are in some cases lines of compression, in 

 other cases lines of extension. 



Of the many discoveries in physical science due to Sir William Grove, 

 the earliest and not the least important is the hattery which hears his name, 

 and is to this day the most powerful of all voltaic arrangements ; but with a 

 Grove's battery of 50 or even 100 cells in vigorous action, the spark will not 

 pass through an appreciable distance of cold air. By using a very large 

 number of cells, carefully insulated and charged with water, IMr. Gassiot 

 succeeded in obtaining a short spark through air ; and lately De La Eue and 

 MiiUer have constructed a large chloride-of-silver battery giving freely sparks 

 through cold air, which, when a column of pure water is interposed in 

 the circuit, accurately resemble those of the common electrical machine. 

 The length of the spark increasing nearly as the square of the number 

 of cells, it has been calculated that with 100,000 elements of this battery 

 the discharge should take place through a distance of no less than eight 

 feet in air. 



In the solar beam we have an agent of surpassing power, the investiga- 

 tion of whose properties by Newton forms an epoch in the history of experi- 

 mental science scarcely less important than the discovery of the law of 

 gravitation in the history of physical astronomy. Three actions chaiacterize 

 the solar beam, or, indeed, more or less that of any luminous body — the 

 heating, the physiological, and the chemical. In the ordinary solar beam 

 we can modify the relative amount of these actions by passing it through 

 dilferent media, and we cau thus have luminous rays with little heating or 

 little chemical action. In the case of the moon's rays it required the highest 

 skill on the part of Lord Eosse, even with all the resources of the observatory 

 of Parsonstown, to investigate their heating properties, and to show that the 

 surface of our satellite facing the earth passes, during every lunation, through 

 a greater range of temperature than the difference between the freezing- and 

 boiling-points of water. 



But if, instead of taking an ordinary ray of light, wc analyze it as Kewton 

 did by the prism, and isolate a very fine line of the spectrum (thcoreticallj' a 

 line of infinite tenuity), that is to say, if wo take a ray of definite refrangi- 

 bility, it will be found impossible by screens or otherwise to alter its pro- 

 perties. It was his clear perception of the truth of this princii)le that led 

 Stokes to his great discovery of the cause of epipolic dispersion, in which he 

 showed that many bodies had the power of absorbing dark rays of high 

 refrangibility and of emitting them as luminous rays of lower refrangibility — 

 of absorbing, in short, darkness and of emitting it as light. It is not, 

 indeed, an easy matter in all cases to say whether a given effect is due to 



