SCIENCE IN THE LAST HALF CENTURY. 87 



KINETIC THEORY OF GASES. 



Tlieoretical and experimental investigatioDS have concurred in the 

 establishment of the view that a gas is a body, the particles of which 

 are in incessant rectilinear motion at high velocities, colliding with one 

 another and bounding back when they strike the walls of the containing 

 vessel ; and, on this theory, the already ascertained relations of gaseous 

 bodies to heat and pressure have been shown to be deducible from 

 mechanical principles. Immense improvements have been effected in 

 the means of exhausting a given space of its gaseous contents ; and 

 experimentation on the i)henomena which attend the electric discharge 

 and the action of radiant heat, within the extremely rarefied media 

 thus produced, has yielded a great number of remarkable results, some 

 of which have been made familiar to the public by the Gieseler tubes 

 and the radiometer. Already these investigations have afforded an 

 unexpected insight into the constitution of matter and its relations 

 with thermal and electric energy, and they open up a vast field for 

 future inquirj' into some of the deepest problems of physics. Other 

 important steps, in the same direction, have been effected by investiga- 

 tions into the absorption of radiant heat proceeding from different 

 sources by solid, fluid, and gaseous bodies. And it is a curious ex- 

 ample of the inter-connection of the various branches of physical 

 science, that some of the results thus obtained have proved of great 

 importance in meteorology. 



SPECTROSCOPY. 



The existence of numerous dark lines, constant in their number and 

 position in the various regions of the solar spectrum, was made out by 

 Frauuhofer in the early imrt of the present century, but more than 

 forty years elapsed before their causes were ascertained and their im- 

 portance recognized. Spectroscopy, which then took its rise, is proba- 

 bly that employment of physical knowledge, already won, as a means 

 of further acquisition, which most impresses the imagination. For it 

 has suddenly and immensely enlarged our power of overcoming the 

 obstacles which almost infinite minuteness on the one hand, and almost 

 infinite distance on the other, have hitherto opposed to the recognition 

 of the i^resence and the condition of matter. One eighteen-millionth 

 of a grain of sodium in the flame of a spirit-lamp may be detected by 

 this instrument; and, at the same time, it gives trustworthy indica- 

 tions of the material constitution not only of the sun, but of the far- 

 thest of those fixed stars and nebulte which afford sufficient light to 

 aflect the eye, or the photographic plate, of the inquirer.* 



ELECTRICAL ADVANCES. 



The mathematical and experimental elucidation of the phenomena of 

 electricity, and the study of the relations of this form of energy with 



