264 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



tension of the water begins to fall is about one-half that at which the 

 camphor movements stopped. The thicloiess is therefore 1.6/2 or 

 0.8 [xix. But Lord Rayleigh gives it as simply 1 [/.pi. 



We may obtain a yet greater precision by a method using drops 

 of a standard solution of oil in a volatile solvent. I prepare a 

 standard solution of oil in pure benzole. I use a solution containing 

 exactly 1 cu. cm. of pure oleine (trioleate of glycerin) per 1,000 

 cu. cm. of benzole and a pipette giving 50 drops of this solution per 

 cubic centimeter. Thus a drop contains ^q^qq of a cubic centimeter 

 of oil, and I place two of these drops upon the water. As soon as they 

 touch the water, the drops spread over the whole surface ; the evapora- 

 tion of the benzole is almost instantaneous and leaves a residue of oleine 

 equal to ^oloo ^^^ 400X10~'^ cu. cm. Earlier measures showed me 

 that this quantity of oil could not cover all the surface of the tray 



Figs. 2, 3. — Arrangement for measuring the limiting thickness of a film of 

 oil; H, film of oil; E, tree water; T I", barrier of powdered talc; BB", 

 hand of paper. 



(625 sq. cm.). I blow upon it to gather the invisible film of oil at 

 the farther end of the tray and then scatter upon the nearer end a 

 light veil of powder with the sieve. The talc thus falls upon the 

 free surface of water E (fig. 2) ; it scatters^ carried by my blowing, 

 but you see it stops abruptly along the barrier TT% which though 

 invisible was sharp, and marks the edge of the oil film H. The 

 stoppage is of striking sharpness. 



I now apply to the portion of water uncovered with oil a band of 

 paper BB' (fig. 3), in order to have a straight capillary border. I 

 now make this barrier approach gently the border of talc which 

 straightens, as is indicated in the figure.^ If the barrier is moved a 

 little farther, the talc grains just at the limit of the oil, and more or 

 less distant from each other because they are slightly oily, we see 



^ It is yet better to collect the talc scattered upon the free surface by the band of 

 papor itself. 



