1879.] Properties of Matter in the Gaseous State. 



319 



impossible to convey any adequate idea without going fully into the 

 subject. 



Some idea of the scope of the investigation may be gathered from 

 the last section in the paper, which is accordingly introduced here. 



Section XIII. — Summary and Conclusion. 



Article 125. The several steps of the investigation which have been 

 described may be enumerated as follows : — 



(1.) The primary step from which all the rest maybe said to follow 

 is the method of obtaining the equations of motion so as to take into 

 account not only the normal stresses which result from the mean 

 motion of the molecules at a point, but also the normal and tangential 

 stresses which result from a variation in the condition of the gas 

 (assumed to be molecular). This method is given in Sections VI, 

 VII, and VIII. 



(2.) The method of adapting these equations to the case of tran- 

 spiration through tubes and porous plates is given in Section IX. The 

 equations of steady motion are reduced to a general equation expressing 

 the relation between the rate of transpiration, the variation of pressure, 

 the variation of temperature, the condition of the gas, and the lateral 

 dimensions of the tube. 



In Section X is shown the manner in which were revealed the pro- 

 bable existence (1) of the phenomena of thermal transposition, and (2) 

 the law of correspondence between all the results of transpiration with 

 different plates, so long as the density of the gas is inversely propor- 

 tional to the linear lateral dimensions of the passages through the 

 plates ; from which revelations originated the idea of making the ex- 

 periment on thermal transpiration and transpiration under pressure. 



(3.) It is also shown in Section X that the phenomena of transjDi- 

 ration resulting from a variation in the molecular constitution of the 

 gas (investigated by Graham) are also to be deduced from the equation 

 of transpiration. 



(4) The method of adapting the equations of motion to the case of 

 impulsion is given in Section XI. 



In Section XII is shown how it first became apparent that the ex- 

 tremely low pressures at which alone the phenomena of the radiometer 

 had been obtained were consequent 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. 



(5.) 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. 



vol. xxviii. 2 A 



i 



