628 PLATEAU ON THE PH^ENOMENA OF A FREE LIQUID MASS 



the mass to the lower ring only, the upper ring being raised as 

 much as possible ; mix the oil intimately, as we have said above ; 

 then depress the upper ring, cause the mass to adhere to it, 

 elevate it so as to form an exact cylinder, and proceed imme- 

 diately to the measurement. 



42. The instrument best suited for effecting the latter opera- 

 tions in an exact manner is undoubtedly that which has received 

 the name of cathetometer, and which, as is well known, consists 

 of a horizontal telescope moving along a vertical divided rule. 

 The distance comprised between the summits of the two seg- 

 ments is first measured by the aid of this instrument ; the distance 

 included between the external planes of the two rings (§ 40) is 

 then measured by the same means. The difference between the 

 first and the second result evidently gives the sum of the two 

 heights, the mean of which must be taken ; and consequently 

 this mean, or the quantity sought, h—f, is equal to half the 

 difference in question. 



The determination of the distance between the external planes 

 of the rings requires peculiar precautions. First, as the points 

 of the rings at which we must look are not exactly at the ex- 

 ternal surface of the figure, the oil interposed between these 



completely restored to it. This is why the oil is withdrawn by the siphon. In 

 fact, the pellicle does not penetrate the interior of the latter, and during its 

 contraction continvies to envelope the small portion remaining; so that after 

 the latter has been removed by the syringe, which ultimately absorbs the pel- 

 licle itself, we get completely rid of the latter. 



Before using the siphon, the thickness and consistence of the pellicle are too 

 slight to enable us distinctly to perceive its presence ; but when the operation 

 of the siphon is nearly terminated, and the mass is thus considerably reduced, 

 we find that the surface of the latter forms folds, hence implying the existence 

 of an envelope. Moreover, when the siphon is removed, the small residuary 

 mass, which then remains freely suspended in the alcoholic liquid, no longer 

 assumes a spherical form, but retains an irregular aspect, appearing to have no 

 tendency to assume any regular form. 



This indifterence to assume figures of equilibrium, arising from a diminution 

 in the liquidity of the superficial layer, constitutes a new and curious proof of 

 the fundamental principle relating to this layer (§§ 6 bis and 10 to 16). 

 M. Hagen (Memoire sur la Surface des Liquides, in the Memoirs of the Aca- 

 demy of Berlin, 1845) has observed a remarkable fact, to which the preceding 

 appears to be related. It consists in this, that the surface of water, left to itself 

 for some time, undergoes a peculiar modification, in consequence of which the 

 water then rises in canillarj' spaces to elevations which are very distinctly less 

 than is the case when its surface is exempt ^ freed from this alteration. This 

 fact might perhaps be explained by admitting that the water dissolves a small 

 proportion of the substance of the solid with which it is in contact, and that the 

 external air acts chemically at the surface of the liquid upon the substance 

 dissolved, thus giving rise to the formation of a slight pellicle which modifies 

 the effects of the molecular forces. 



