745 



Instead, however, of (he ealcnlatioii of (lie forces appearing in 

 case of collision, the friction through this cause can also be calcu- 

 lated bj means of the momentum that at impact is niomentaneousl}' 

 conveyed from the centre of one of the colliding molecules to that 

 of the other. This method of calculation seems simpler and will 

 be carried out in ^ 2. 



II. Friction for (loiihle points. Formation of strr((ks. We might also 

 assume the molecules lo be electrical or maguetical double points. 

 When they were orientated with regai-d to each other (piite arbi- 

 trarily, they would equally fiequeutly I'epel as attract each other, 

 so that the mean force would be zero. Through the cou|)les which 

 they exert on each other, they will, however, turn so that attraction 

 [)revails. When we now assume that niolecules that approach each 

 other, are still little orientated, whereas this is the case to a higher 

 degree with molecules that have moved past each other, and recede 

 again from each other, the molecules whose central line lies in the 

 ^•'-direction will be more orientated on an average than those for 

 which it lies in the j' direction, so that a traction in the ./' direction 

 will result, greater than in the e'-direction, which can again account 

 for the tensor of tension. 



When we consider more than two molecules whose centres lie 

 on the same line in (he ./'-direction, the cou{)les they exert on each 

 other, will strengthen each othei", which can give rise to the forma- 

 tion of a kind of streaks, which still moi-e promotes the friction. 



it is ditTicnlt to compute the accurate amonnt of this orientation 

 of the molecule axes; it will be different according as one thinks 

 the rotations of the molecules determined by classical mechanics 

 or by the laws of the theoi-y of quanta. Besides there is no occasion 

 in the experimental data to assume that this case actually presents 

 itself. I shall, therefore, not attempt to calculate the friction according 

 to this hypothesis, though possibly it plays a decisive part in the 

 friction of exceedingly \ iscous liquids, which present themselves as 

 bi-refringent in case of friction, as likewise in the glassy state. 



III. Friction in consequence of foriitation of y roups. Finally we 

 can assume the molecules (o combine to groujjs in consequence of 

 their mutual attraction. In li([uids at rest these groups will possess 

 spherical symmetry on an average. When, however, a liquid is in 

 a motion for which n = ((z, these spherical groups will be elongated 

 to ellipsoids. This variation of shape will now again give rise to a 

 greater traction in the .^'-direction. This cause of tViction will 

 probably chietly make itself felt iu the neighbourhood of (he critical 

 point. In § 4 and following paragra[)hs 1 will make an attempt 



