885 



of bubbles and drops by Quinckk, Magie and Wit.beri'ouck, 3) detei'- 

 niinalioii by means of ripples (Lord Ravi.kioh Pliil. Mag-. XXX 

 p. 386), (4) Lknaku's (Wied. Ann. XXX p. 209) nielbod of determ- 

 ination by oscillations of a spherical drop of liqnid, (5) determination 

 by the size of drops (Rayleigh Phil. Mog. 48, p. 321) (6) Wilhelmy's 

 method of measuring the downward [)nll exerted by a liqnid on a 

 thin plate of glass or metal partly imuieised in the li(jni(l, (7) .lAE(iEK's 

 method of measuring the least |)ressure which will furce bubbles 

 of air from the narrow oritice of a capillary tube dipping into the 

 liquid, (8) by measuring the pull required to drag a plate of known 

 area away from the surface of a licpiid etc. 



Besides (v/) the surface tension, another constant is sometimes 

 employed ; it is called specific cohesion, and is usually denoted by 

 a'. The relation i)etween a' and (y) is expressed as follows: 



2t/ 

 r/° ^ ^^ ^ specific cohesion, where ^/ =: density of the li(piid, wlieiice 

 d 



since {y)^^l^rlid (where /'^i-adius of the capillary tube, /< = 

 = rise in the tube), it is seen that the specilic cohesion is measured 

 by the height to which a liipiid rises in a ca|)illary tniie of unit 

 radius. 



Walden [Zeit. Phys. (Jhem. 65, 129, 257 (1«(J8)J has recently 

 found that specific cohesion may be applied in another way to esti- 

 mate the degree of association of both liquids and solids. A com- 

 parison of the expei-imenlal data showed the relationsiiip 



2\ 



:= couslaut ■= 17.9 



,2 



where 7'c; is the latent heal of va|)Oiisation at a boiling point and 

 (7%, the specific cohesion at the same tenqierature. Combining this 

 expression with Tkouton's rnle, we see that ihe molecidar cohesion 

 of a liquid at its boiling point is proportional to the boiling temper- 

 ature expressed on the absolute scale. This relaiion holds only for 

 non-associated liquids. 



Moreover Walden points out that if substances ai-e in corresponding- 

 states at their melting [)oints, there would be a similar relation 

 between the latent heat of fusion and the specific cohesion at the 

 melting point. 



The specific cohesion of fused metals and salts has been investi- 

 gated by (Quincke in a very thorough manner. The measurements 

 were obtained from the weight of falling drops of a liquid, or from 

 the curvature of flat drops of the solidified material. 



It was found (Pogg. Ann. 135, 643, IStïH) that all salts and 

 metals and .some organic substances near their melting points have 



