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Prof. 0. Reynolds on certain Dimensional [Feb. 6*, 



(6.) Section II, Part I, contains a description of the experiments 

 undertaken to verify the revelations of Section X respecting thermal 

 transpiration, which experiments establish not only the existence of 

 the phenomena, but also an exact correspondence between the results 

 for the different plates at corresponding densities of the gas. 



(7.) Section III contains a description of the experiments on trans- 

 piration under pressure undertaken to verify the revelations of Sec- 

 tion X with respect to the correspondence between the results to be 

 obtained with plates of different coarseness at certain corresponding- 

 densities of gas, which experiments proved not only the existence of 

 this correspondence, but also that the ratio of the corresponding 

 densities in these experiments is the same as the ratio of the corre- 

 sponding densities with the same plates in the case of thermal trans- 

 piration — a fact which proves that the ratio depends entirely on 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 reve- 

 lations of Section XII, from which experiments it appears that, 

 with these small surfaces, phenomena of impulsion, similar to those of 

 the radiometer, occur at pressure but little less than that of the 

 atmosphere. 



Conclusion. 



Article 126. As regards transpiration and impulsion, the investiga- 

 tion appears to be complete ; most, if not all, the phenomena pre- 

 viously known have been shown to be such as must result from the 

 tangential and normal stresses consequent on a varying condition of a 

 molecularly constituted gas ; while the previously unsuspected phe- 

 nomena to which it was found that a variation in the condition of gas 

 must give rise, have been found to exist. 



The results of the investigation lead to certain general conclusions 

 which lie outside the immediate object for which it was undertaken; 

 the most important of these, namely, that gas is not a continuous 

 plenum, has already been noticed in Article 5, Part I. 



The Dimensional Properties of Gas. 



Article 127. The experimental results considered by themselves 

 bring to light the dependence of a class of phenomena on the relations 

 between the density of the gas and the dimensions of the objects 

 owing to the presence of which the phenomena occur. As long as 

 the density of the gas is inversely proportional to the coarseness of the 

 plates the transpiration results correspond ; and in the same way, 

 although not so fully investigated, corresponding phenomena of im- 

 pulsion nre obtained as long as the density of the gas in inversely 

 proportional to the linear size of the objects exposed to its action ; 



