PROPERTIES OF MATTER IN THE GASEOUS STATE. 
731 
the different plates bear a fixed ratio; and this is the case whatever may be the cause of 
the transpiration, i.e., a difference of temperature or a difference of pressure (a difference 
of gas I have not investigated, as it was obviously unnecessary to do so). Thus with 
two plates, one of stucco and the other of meerschaum, similar results of transpiration 
caused by pressure were obtained when the densities with the plates were respectively 
as 1 to 5'6, both with hydrogen and air and at pressures ranging from 30 to 2 inches 
of mercury. Also with the same two plates similar results of thermal transpiration 
were obtained when the densities were respectively as 1 to 6'5 both for ah and 
hydrogen, and through a range of pressures from 30 to ‘25 inches of mercury. The 
discrepancies of 5'6 and 6'5 were in all probability owing to a slightly altered 
condition of the plates (see Appendix, note 4). 
This correspondence of the results at corresponding densities holds, although the law 
of motion changes. Thus with air at 30 inches the law was the same as that obtained 
by Graham for stucco plates, while at the smallest pressures (‘25 inch) it was nearly 
the same as he found for graphite plates or apertures in thin plates. 
4. Having established this law of corresponding results at corresponding densities, it 
became apparent that the results obtained with plates of different coarseness and with 
the same plates but different densities of gas also followed a definite law. This law, 
which admits of symbolical expression, shows that there exists a definite relation 
between the results obtained, the lateral dimensions of the passages, and the density 
of the gas. 
This law is important as reconciling results which have hitherto appeared to be 
discordant, such as Graham’s results with plates of different coarseness, and as tending 
to complete the experimental investigation; but it has another and a more general 
importance. 
It may not appear at first sight, but on consideration it will be seen that this law 
amounts to nothing less than an absolute experimental demonstration that gas 
possesses a heterogeneous structure—that it is not a continuous plenum of which each 
part into which it may be divided has the same properties as the whole. 
It would appear that Graham must have had this proof, so to speak, under his eyes, 
and it is strange that both he and subsequent observers have overlooked it. It seems 
possible, however, that they were not alive to the importance of such a demonstration. 
It is now so generally assumed that gas does possess molecular structure that the 
weakness of the evidence on which the assumption is based and the importance of 
further proof are points that are apt to escape notice. 
The importance of an experimental demonstration that gas possesses 
molecular structure. 
5. The idea of molecular gas does not appear to have originated from the recognition 
of properties of gas which were inconsistent with the idea of a continuous plenum, but 
from a wish to reconcile the properties of gas with the properties of other substances, 
