Fan. 5, 1882] 
signalised and briefly discussed. This preliminary sketch is 
wound up by a reference to arecently- published paper by Lecher 
and Pernter, who, while supporting the lecturer in the matter of 
gases, dissent from him in the matter of vapours. These investi- 
gators are especially emphatic in affirming the neutrality of 
aqueous vapour to radiant heat. Following Magnus, they refer 
Tyndall’s results to what Magnus calls ‘‘vapour-hesion,” that 
is to say, to the condensation of the vapours on the surfaces of 
the plates of rock-salt used to close the experimental tube, and 
on the interior surface of the tube itself. 
In Noyember, 1880, the lecturer’s investigations in this field 
were resumed. Former experiments were repeated and verified 
with divers sources of heat, and with various experimental tubes 
—some polished within, and others coated inside with lampblack. 
The results obtained with the one class of tubes are substantially 
the same as those obtained with the other. 
But even a coating of lampblack may be supposed to reflect a 
certain amount of heat, hence the desirability of an arrangement 
whereby internal reflection should be entirely abolished. This 
was accomplished in the following manner :—A spiral of 
platinum wire, rendered incandescent by a voltaic current of 
measured strength, was chosen as source of heat. An experi- 
mental tube 38 inches long and 6 inches in diameter had two 
circular apertures at its ends, closed by transparent plates of 
rock-salt, 3 inches in diameter, The tube was furnished with 
three cocks—one connected with a large Bianchi’s air-pump ; 
another with a purifying apparatus; while through the third 
vapours and gases could be admitted. Prior to entering the tube 
the calorific rays were sent through a very perfect rock-salt lens, 
by means of which an image of the spiral was formed on the 
most distant plate of rock-salt. To obtain the image with 
clearness, the spiral was first rendered highly luminous, and 
afterwards reduced, by the introduction of resistance, to the 
required temperature. In this way a calorific beam was sent 
along the axis of the experimental tube without at all impinging 
upon its interior surface. No reflection came into play ; no ab- 
sorption by hypothetical liquid films, coating the internal 
surface, could occur; and yet experiments made with this 
arrangement entirely confirmed the preceding ones, wherein by 
far the greater quantity of heat which reached the pile had 
undergone reflection. 
When the source of heat was changed to a carefully-worked 
cylinder of lime, a portion of which was rendered incandescent 
by an ignited stream of coal-gas and oxygen, the results were 
confirmatory of those obtained with the spiral. The order of 
absorption in both cases was the same, the only difference being 
that the fractional part of the total radiation absorbed in the 
case of the lime-light was less than that absorbed in the case of 
the spiral. 
To condense the radiation from the lime-light, concave mirrors 
were sometimes employed, and sometimes rock-salt lenses. The 
results in both cases were identical. 
An experimental tube of the dimensions here given was em- 
ployed by the lecturer to check his results more than ten years 
ago. Its interior surface was rough and tarnished, and when 
warmed dynamically by the entrance of a gas its power as a 
radiator enabled it to disturb, to some slight extent, the purity of 
the results. To obviate this, the experimental tube recently em- 
ployed was provided with an internal silver surface, deposited 
electrolytically and highly polished. By this arrangement the 
radiation of the tube itself, as well as its absorption, was ren- 
dered quite insensible. 
The rock-salt plates used to close the experimental tube, and 
on which liquid films are alleged to be deposited, remain to 
be examined. In this case also an exferimentum crucis is 
possible. If the observed absorptions be due to such liquid 
films, then the separation of the salts more widely from each 
other, the space between them being copiously supplied with 
vapour, ought to produce no effect ; but if the absorption, as 
alleged by the lecturer, be the act of the vapour molecules, then 
the) deepening of the absorbing stratum ought to produce an 
augmented effect. For many gases and some vapours this 
problem was solved as far back as 1863. By means of an appa- 
ratus then described, polished plates of rock-salt could be 
brought into contact with each other, and then gradually sepa- 
rated, until the gaseous stratum between them was some inches 
in depth. With sulphuric ether vapour, the distance between 
the plates being one-twentieth of an finch, an absorption of 2 
per cent, was observed. With a thinner stratum, or a weaker 
vapour, even this small absorption vanished, while in passing 
NATURE 
233 
from one-twentieth of an inch to two inches the absorption rose 
from 2 per cent. to 35 per cent. of the total radiation, Such 
experiments, recently verified, entirely dispose of the hypothesis 
that liquid films were the cause of the observed absorption. 
The ‘‘vapour-hesion hypothesis” involves the assumption that 
liquids exert on radiant heat an absorbent power which is denied 
to their vapours. It assumes, in other words, that the seat of 
absorption is the molecule considered as a whole, and not the 
constituent atoms of the molecule. For were the absorption 
intra-molecular, the passage from the liquid to the vaporous 
condition, which leaves the molecules intact, could not abolish 
the absorption. So far back as 1864 the lecturer had proved 
that when vapours, in quantities proportional to the densities of 
their liquids, were examined in the experimental tube, the order 
of their absorptions was precisely that of the liquids from which 
they were derived. This result has been recently tested and 
verified in the most ample manner by means of the apparatus in 
which internal reflection never comes into play, It furnishes, 
therefore, the strongest presumptive evidence that the seat of 
absorption in liquids and in vapours is the same. 
As a problem of molecular physics it was, however, in the 
highest degree desirable to compare together egwa/ quantities, 
instead of proportional quantities, of liquids and vapours. 
Highly volatile liquids alone lend themselves to this experiment, 
for only from such liquids can vapours be obtained sufficient, 
when caused to assume the liquid form, to produce layers of 
practicable thickness. Two cases, however, have been very 
fully worked out, the substances employed being the hydride of 
amyl and sulphuric ether. Careful and exact experiments, many 
times repeated, lead to the result that when the number of mole- 
cules traversed by the calorific rays in the vapour is the same as 
that traversed in the liquid, the absorptions are identical. In 
the silvered experimental tube, which, as stated, is 38 inches 
long, hydride of amyl vapour, at a mercury pressure of 6°6 
inches, is equivalent to a liquid layer 1 millim. in thickness, 
while a vapour column of sulphuric ether, of the same length, 
and 7*2 inches pressure, would also produce a liquid layer 1 
millim. thick. The experiment has been made with the utmost 
care, both with the lime-light and the incandescent platinum, 
with the result that it is impossible to say that there is any dif- 
ference between the vapour absorption and the liquid absorption. 
In the face of such facts the ‘‘ vapour-hesion ” hypothesis, as an 
explanation of the results published by the lecturer, cannot be 
sustained, 
On November 29, 1880, he had the pleasure of witnessing, in 
the laboratory of the Royal Institution, the experiments of Mr. 
Graham Bell, wherein a concentrated luminous beam, rendered 
intermittent by a rotating perforated disk, was caused to impinge 
upon various solid substances, and to produce musical sounds, 
Mr. Bell’s previous experiments upon selenium naturally led him 
to conclude that the effect was produced by the luminous rays of 
the spectrum. The contemplation of these experiments pro- 
duced in the lecturer the conviction that the results were due to 
the intermittent absorption of radiant heat. He was experi- 
menting on vapours at this time. Substituting in idea gaseous 
for solid matter, he clearly pictured the sudden expansion of an 
absorbent gas or vapour at every stroke of the calorific beam, 
and its contraction when the beam was intercepted. Pulses far 
stronger than those obtainable from solid matter would probably 
be thus produced, which, when rapid enough, would generate 
musical sounds. ‘The intensity of the sound would, of course, 
be determined by the absorptive power of the gas or vapour. 
This idea was tested on the spot. Placing sulphuric ether in a 
test-tube, and connecting the tube with the ear, the intermittent 
beam was caused to fall upon the vapour above the liquid. A 
feeble musical sound was distinctly heard. Formic ether was 
tried in the same way, and with the same result. Bisulphide of 
carbon was then tried, but the vapour of this liquid proved in- 
competent to generate a musical sound. These results, which 
were in perfect accordance with those previously enunciated by 
the lecturer, were first made public during a discussion at the 
Society of Telegraph Engineers on December 8, 1880 (Yournal 
of Telegraph Engineers, vol. ix. p. 382). 
It was obvious, however, that the arrangement of Mr. Bell— 
a truly beautiful one—was not suited to bring out the maximum 
effect. He had employed a series of lenses to concentrate his 
beam, and these, however pure, would, in the case of trans- 
parent gases, absorb a large portion of the rays most influential 
in producing the sound, The lecturer, therefore, resorted to 
lenses of rock-salt and to concave mirrors silyered in front, He 
