316 
PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
radiant heat to enter T T', the pre-existing equilibrium is destroyed; the needle 
moves promptly away from zero, and from the observed deflection the absorption may 
be calculated. 
Instead of the concave reflector, a rocksalt lens of great purity is sometimes used to 
render the beam parallel. When the lens is used, the incandescent portion of the 
cylinder of lime is caused to face the tube T T', between which and the lime the lens is 
introduced. Instead of the incandescent lime, a spiral of platinum wire heated to 
redness by a voltaic current is frequently employed as the source of heat. The spiral 
is placed in the centre of a brass globe G, fig. 4, and is connected with the screws S S' 
Fig. 4. 
T 
fixed in the back of the globe, from which wires pass to the battery. It shows 
the position of the rocksalt lens, while one end of the experimental tube is shown at T, 
The greatest care is here necessary to protect the spiral from any agitation of the air. 
Such care, however, it is always in the experimenter’s power to bestow. To secure 
constancy in the radiation, the battery was charged three times a day, the strength of 
the current being regulated by a rheostat, and checked by a tangent galvanometer. 
An experimental tube of the description shown in fig. 3 was first employed by me 
about twelve years ago, and with it I then verified all the experiments I had previously 
made on the absorption of radiant heat by gases and vapours. The experiments were 
not published in any scientific journal, but they are thus referred to at page 394 of 
my volume of collected memoirs, entitled “ Contributions to Molecular Physics in the 
domain of Radiant Heat” :—“ The two ends of an experimental tube 38 inches long 
and 6 inches in diameter were each provided with an aperture 2‘6 inches in diameter. 
These apertures were closed with plates of rocksalt. The source of heat was a 
platinum spiral, well defended from air-currents, and heated to redness by an electric 
current. In front of the spiral was a rocksalt lens, which sent a slightly convergent 
beam through the tube, Behind the most distant plate was formed a sharply defined 
