82 SCIENTIFIC APPARATUS. 



under strain. The continuity of the so-called three forms of 

 matter, Solid, Liquid, and Gaseous, is supported by the observation 

 that while both solids and liquids have cohesion, both liquids and 

 gases have viscosity, and all three have volume elasticity. While 

 the cohesion of solids is measured directly by their resistance to 

 rupture, that of liquids may be determined either in the same way 

 or by the size of a drop, which on reaching a certain stage of its 

 growth detaches itself from its root, or indirectly by pitting liquid 

 cohesion and gravity against the adhesion between the liquid and 

 a solid in a capillary tube. The viscosity of liquids and gases, 

 that is, their resistance to intermolecular disturbance, is manifested 

 by the obstruction they offer to solids moving through them. 

 And this effect is, in the case of liquids, complicated by the 

 adhesion between the solid and liquid. The transpiration of gases 

 through narrow tubes, and the flow of liquids through them, are 

 also conditioned by, and consequently measure, fluid viscosity. 



The intermolecular motion of a single fluid apart from the 

 purely hypothetical heat agitation is exhibited in sound waves or 

 travelling variations in density. Here the molecule moves only in 

 a straight line passing through the origin of commotion. And in 

 such molecular motion the only variation in one and the same 

 fluid appears to be (i) wave length or distance between points in 

 homologous phase ; (2) wave amplitude, or length of molecular 

 excursion ; and (3) probably wave quality, or kind of variation in 

 density between neighbouring homologous points. Since sound 

 waves of all amplitudes travel nearly at the same rate the fre- 

 quency of recurrence of wave impact at a point at a constant dis- 

 tance from the wave source the audible pitch is a measure of 

 the wave length ; while the loudness is the physiological measure 

 of the wave amplitude. The total density of a mass of air receiv- 

 ing and transmitting sound waves is less than when it is at rest, 

 and there is accordingly a tendency of bodies which obstruct the 

 wave system to approach the source of sound. 



More various in kind are the systematic and sequential mole- 



