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POPULAR SCIENCE REVIEW. 
sensation of heat, the motions of the particles of the hot body 
are imparted to our nerves, and by friction the particles 
of bodies are caused to assume that peculiar motion which 
announces itself to our senses as heat. 
It is, however, well known that heat may pass through 
a vacuum, and that the sun’s rays pass, on their way to 
our globe, through immense spaces considered void of matter. 
How is heat, according to the dynamical theory, transmitted 
through a vacuum? In order to account for such trans- 
mission, the whole of the universe and the pores of all bodies 
are supposed to be filled with a medium of extreme tenuity and 
almost infinite elasticity, called ether. A sounding bell causes 
the elastic air to vibrate, and when the vibrations reach our 
ears we obtain the sensation of sound. In an analogous 
manner, a hot substance induces vibrations in the ether, which 
spread in all directions with immense velocity along straight 
lines, until their progress is stopped or their direction changed 
by some obstacle in their path. This sort of motion, called 
radiant heat, may take place in a vacuum, because the ether is 
supposed to be everywhere present. If the heat-waves strike 
against a substance placed in their way, their motion is 
imparted to the atoms of the body, and thus the temperature of 
the substance is raised, or they are reflected, or they are allowed 
to pass through the substance. The reflection, absorption, and 
transmission of radiant heat in solid and liquid bodies has 
been examined by the illustrious Melloni ; he also attempted 
to ascertain by experiment if gases do absorb sensible 
quantities of radiant heat. Kays, allowed to pass through 
a column of air 18 feet long, did not exhibit any diminution 
in their intensity. On the other hand. Dr. Franz, of Berlin, 
found that air contained in a tube three feet long absorbed 
about 3*5 per cent, of the heat radiated from an Argand 
lamp. Both results are incorrect; Melloni’ s apparatus was not 
sensitive enough, and Franz’s arrangement gave results which 
were not at all due to absorption. 
Except one experiment, by Sir H. Davy, these are, we 
believe, the only attempts made to measure directly the 
transmission and absorption of heat by gases. If gases 
absorb heat at all, more decided and valuable results may be 
expected from their examination than either from solids or 
liquids. In gases we have not to contend with disturbing 
influences, such as the force of cohesion, and therefore the 
laws which govern radiation and absorption will reveal them- 
selves in more simple forms. Also the influence of our 
atmosphere on solar and terrestrial radiation would become 
known in such an inquiry. 
The most obvious way to determine experimentally the 
