PROPERTIES OF MATTER IN THE GASEOUS STATE. 
735 
the gas is sufficiently dense, be inversely proportional to the density ; but as the 
rarefaction increases, the increase in the time of transpiration becomes less and less, 
until the time becomes constant. 
Law VII.—When the rarefaction is so great that the time of transpiration of equal 
volumes of the same gas is constant, the times of transpiration of equal volumes of 
different gases will be proportional to the square root of the atomic weights of 
the gases. 
Respecting the results of impulsion and the connexion between these results and the 
relation between the density of the gas and the size of the vanes. 
Law VIII.—When the gas is sufficiently dense, then the impulsive force will be 
inversely proportional to the densities of the gas; but as the rarefaction proceeds the 
increase in the force becomes less and less until the rarefaction has reached a point 
depending on the size of the vanes (the larger the vanes the higher must be the 
rarefaction), after which the force begins to diminish, and ultimately diminishes with 
the density. 
These laws were reduced to the form in which they have been stated in order to 
adapt them for experimental verification. Thus they do not represent the simplest 
nor yet the fullest form in which the properties of the gas can be expressed. This 
may be seen by reference to Sections X. and XII. which treat of the theory of these 
properties. There definite expressions will be found for the relations indefinitely 
indicated in Laws I. and II. These definite expressions are not introduced here, 
because they have not been definitely verified by experiment. 
The definite relations expressed in Laws III., IV., V., VI., VII., and VIII., although 
derived from theoretical considerations, have all been to a greater or less extent veri¬ 
fied by experiment—as far as the possible range of densities would admit—and in all 
cases the experimental results have within the limits of error corresponded well with 
the theoretical deductions. 
Section II.— Experiments relating to Thermal Transpiration. 
10. In commencing these experiments it was impossible to form any estimate 
whatever of the magnitude of the results to be expected. The laws just stated only 
showed what would be the comparative value of the results under different circum¬ 
stances ; so that until a result had been found it was impossible to predict whether, 
with any particular plate, the result would be appreciable or not. 
Thus it happened that although the experiments commenced on January 15, 1878, 
it was not until March that any definite results were obtained. This delay was 
chiefly owing to several very subtle sources of disturbance, the effect of which could 
only be distinguished from true results after a series of tests extending in each case 
over several days. 
MDCCC'LXXIX, 
5 B 
