PROPERTIES OF MATTER IN THE GASEOUS STATE. 
841 
the plate ; from which revelations originated the idea of making experiments on 
thermal transpiration and transpiration under pressure. 
(3.) The method of adapting the equations of steady motion to the case of impulsion 
is given in Section XI. 
In Section XII. is shown how it first became apparent that the extremely low 
pressures at which alone the phenomena of the radiometer had been obtained were con¬ 
sequent on the comparatively large size of the vanes, and that by diminishing the size 
of the vanes similar results might be obtained at higher pressures ; whence followed 
the idea of using the fibre of silk and the spider-line in place of the plate-vanes. 
(4.) In Section XII. it is also shown that while the phenomena of the radiometer 
result from the communication of heat from a surface to a gas, as explained in my 
former paper, these phenomena also depend on the divergence of the lines of flow; 
whence it is shown that all the peculiar facts that have been observed may be 
explained. 
(5.) In Section X. it is also shown that the phenomena of transpiration, resulting 
from a variation in the molecular constitution of the gas (investigated by Graham), are 
also to be explained by the equation of transpiration. 
(6.) Section II. (Part I.) contains a description of the experiments undertaken to 
verify the revelations of Section X. respecting thermal transpiration; which experi¬ 
ments establish not only the existence of the phenomena, but also an exact correspon¬ 
dence between the results for different plates at corresponding densities of the gas. 
(7.) Section III. contains a description of the experiments on transpiration under 
pressure, undertaken to verify the revelations of Section X. with respect to the 
correspondence between the results to be obtained with plates of different coarseness 
at certain corresponding densities of the gas ; which experiments proved, not only 
the existence of this correspondence, but also that the ratio of the corresponding den¬ 
sities in these experiments are the same as the ratio of the corresponding densities 
with the same plates for thermal transpiration—a fact which proves that the ratio 
depends on the relative coarseness of the plates. 
(8.) Section IV. contains a description of the experiments with the fibre of silk and 
with the spider-line, undertaken to verify the revelations of Section XII. ; from which 
experiments it appears that, with these small surfaces, phenomena of impulsion similar 
to those of the radiometer occur at pressures but little less than that of the atmo¬ 
sphere. 
126. As regards transpiration and impulsion, the investigation appears to be com¬ 
plete. Most, if not all, the phenomena previously known have been shown to be such 
as must result from the tangential and normal stresses consequent on a varying 
condition of molecularly constituted gas ; while the previously unsuspected phenomena 
to which it was found that a variation in the condition of a molecular gas must give 
rise, have, on trial, been found to exist. 
The results of the investigation lead to certain general conclusions which lie outside 
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