1879.] 



Properties of Matter in the Gaseous State. 



315 



this point, the fibre moved away from the heat, and the motion 

 steadily increased as the pressure became very small, snch as -g^th 

 of an inch of mercury. 



With hydrogen in the test-tube, the fibre moved away from the heat 

 at all pressures below that of the atmosphere, and for small pressures 

 the motion was somewhat larger than with air. 



A spider-line was also tried, and gave results similar to the fibre of 

 silk. 



It thus appeared that both with the fibre of silk and the spider-line 

 the phenomena of impulsion were manifest at densities many hundred 

 times greater than the highest densities at which like results are 

 obtained with the vanes of the radiometer which are several hundred 

 times broader than the fibre of silk. And this verifies the law of 

 corresponding results at corresponding densities for this class of phe- 

 nomena. 



Abstract of Part II (Theoretical Investigation). 



The characteristic as well as the novelty of this investigation is 

 that, not only is the mean of the motions of the molecules at the 

 point under consideration taken into account, but also the manner in 

 which the mean motion may vary from point to point, in any direction 

 across the point under consideration. It appears that such a variation 

 gives rise to certain stresses in the gas (tangential and normal) and it 

 is of these stresses that the phenomena of transpiration and impulsion 

 afford evidence. 



Instead of considering only the mean condition of the molecules 

 comprised within an elementary volume of the gas, what is chiefly 

 considered in this investigation is the mean condition of the molecules 

 which cross an elementary area in a ^ plane supposed to be drawn 

 through the point. 



Q is used to indicate a quantity belonging to a molecule such as its 

 mass, momentum, or energy, ^(Q) to express the rate at which Q is 

 carried across a plane perpendicular to the direction x. 



Two systems of axes are employed, xyz fixed axes, with respect 

 to which u, v, w, are the component velocities of a molecule, and a 

 system of axes parallel to xyz, but moving with the component 

 velocities U, Y, W. with respect to which 77, have the com- 

 ponent velocities of a molecule. U, Y, W, having such values that 



As preliminary to the investigation, expressions are obtained for 

 <t(Q) in terms of o, U, Y, and W, on the supposition that the gas is 

 uniform. This is accomplished by the application of well-known 

 methods. 



