PROGRESS DURING THE NINETEENTH CENTURY. 
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continue to be made and fresh elements discovered, such as argon 
by Lord Bayleigh in 1894, and helium by Ramsay; and we now 
know that probably all elements can exist either as gases, liquids, 
or solids. In 1899 this was demonstrated as regards that very light 
gas hydrogen, which after having been converted into a liquid was 
at last frozen solid at a temperature of —267° Centigrade, that is 
to say, nearly as much colder than ice, as melting lead is hotter. 
(Lead melts at 320° Cent., and what is known as the absolute zero 
is -273° Cent.) 
The past century has also taught us much about light, heat, and 
electricity. As regards light, it opened with the discovery of the 
laws of interference by Young in 1801. Polarization was noticed 
and explained by Malus in 1810, and about this period the inde¬ 
pendent investigations of Dr. Wollaston and Fraunhofer called 
attention to spectrum analysis and the spectroscope, that most 
important of all aids to the chemist and astronomer, and one with 
which the names of Kirchhoff, Bunsen, Huggins, and Lockyer are 
closely connected. 
At the beginning of the century the old idea that heat or caloric 
existed as an actual body always found in matter, was being 
gradually disproved, “phlogiston” and “caloric” dying hard 
together. But probably the most important event of the century 
as regards heat is Joule’s determination of its mechanical equivalent, 
about 1845, a discovery which has been of such vital importance in 
the development of the steam-engine. 
The enormous advance made in electrical knowledge during this 
period is obvious to everyone. In 1790 Galvani had produced 
electricity by the contact of metals with fluids, and in 1800 Yolta 
produced it by the contact of different metals; but, when the last 
century opened, galvanic electricity, frictional electricity, and 
magnetism were still regarded as independent of each other, and 
it was not until 1820, after Oersted had shown the connection 
between electricity and magnetism, that the three were considered 
to be one. Ampere, Joseph Henry, and Faraday greatly advanced 
matters, and the latter in 1830 found that a magnet could produce 
electric currents in wires, and in 1848 showed that polarized light 
was affected by electricity. Towards the end of the century 
further great steps in electrical knowledge have been made. Hertz 
has measured the length of electric waves, and demonstrated that 
electricity can be reflected, refracted, and polarized. And Maxwell 
has proved that heat, light, and electricity are essentially one, and 
differ only in their wave-lengths. 
The electric telegraph, which was foreshadowed at the end of 
