328 PROFESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 
last five years. In these researches radiant heat has been used as an instrument for 
exploring molecular condition, and this is the object which I have kept constantly in 
view throughout the investigation which I have now the honour to submit to the Royal 
Society. 
The first part of these researches is devoted to the more complete examination of 
a subject which was briefly touched upon at the conclusion of my Fourth Memoir — 
namely, the action of liquids, as compared with that of their vapours, upon radiant 
heat. The differences which exist between different gaseous molecules, as regards 
their power of emitting and absorbing radiant heat, have been already amply illus- 
trated. When a gas is condensed to a liquid, the molecules approach and grapple 
with each other by forces which are insensible as long as the gaseous state is main- 
tained. But though thus condensed and enthralled, the ether still surrounds the mole- 
cules. If, then, the powers of radiation and absorption depend upon them individually, 
we may expect that the deportment towards radiant heat which experiment establishes 
in the case of the free molecule, will maintain itself after the molecule has relinquished 
its freedom and formed part of a liquid. If, on the other hand, the state of aggrega- 
tion be of paramount importance, we may expect to find on the part of liquids a deport- 
ment altogether different from that of their vapours. 
Melloni, it is well known, examined the diathermancy of various liquids, but he 
employed for this purpose the flame of an oil-lamp, covered by a glass chimney. His 
liquids, moreover, were contained in glass cells ; hence the radiation from the source 
was profoundly modified before it entered the liquid at all, for the glass was imper- 
vious to a considerable part of the radiation. It was my wish to interfere as little as 
possible with the primitive emission, and also to compare the action of liquids with 
that of their vapours, when examined in a tube stopped with plates of rock-salt. I 
therefore devised an apparatus in which a layer of liquid of any thickness could be 
enclosed between two polished plates of rock-salt. It was skilfully constructed for me 
by Mr. Beckek, and the same two plates have already done service in more than six 
hundred experiments. 
The apparatus consists of the following parts : — A B C (fig. 1) is a plate of brass, 
3-4 inches long, 2T inches wide, and 0 - 3 of an inch thick. Into it, at its corners, are 
rigidly fixed four upright pillars, furnished at the top with screws, for the reception of 
the nuts qrst. D E F is a second plate of brass of the same size as the former, and 
pierced with holes at its four corners, so as to enable it to slip over the four columns 
of the plate ABC. Both these plates are perforated by circular apertures, m n and 
op, l 1 35 inch in diameter. GHI is a third plate of brass of the same area as DEF, 
and, like it, having its centre and its corners perforated. The plate G H I is intended 
to separate the two plates of rock-salt, which are to form the walls of the cell, and its 
thickness determines that of the liquid layer. Thus when the plates ABC and DEF 
are in position, a space of the form of a shallow cylinder is enclosed between them, and 
this space can be filled 'with any liquid through the orifice Jc. The separating plate 
