364 Lord Rayleigh on Experiments 



portion is rejected. Operating in this way there was no diffi- 

 culty in obtaining surfaces upon which a drop of water would 

 spread, although, from causes that could not always be traced, 

 a certain proportion of failures was met with. 



Exposure of the glasses to the atmosphere soon tells upon 

 the success of the experiment, although on one occasion 

 spreading occurred after a glass had stood (with protection 

 from dust) for 20 hours. Even so short an exposure as 

 10 minutes was found to prejudice the condition of the 

 mercury surface. Although something here may have de- 

 pended upon the special character of the sample of mercury, 

 it will be advisable in repeating the experiment to pour the 

 mercury at the last moment. 



As might be expected, the grease which produces these 

 effects is largely volatile. In many cases a very moderate 

 preliminary warming of the watch-glass makes all the differ- 

 ence in the behaviour of the drop. 



So far as I have observed, the spreading of the drop takes 

 place always in a leisurely fashion. If a little powder of 

 recently ignited magnesia be dusted over the mercury, there 

 is no violent repulsion of the dust before the advancing water. 

 But if a small drop of oil be substituted for the water, the 

 powder is flashed away so quickly that the eye cannot follow 

 the operation. The difference between the two cases appears 

 to depend upon the atmospheric moisture. As soon as the 

 mercury is poured, it coats itself wdth an aqueous film, and the 

 subsequent spreading of the drop takes place upon a surface 

 whose affinity for water is already largely satisfied. A drop 

 of water that has spread and then partially gathered up again 

 (as usually happens after a short interval) shows an interesting 

 behaviour when breathed upon. The disk contracts some- 

 what, and then as the breath, which need hardly be visible, 

 passes off, expands again ; and thus a number of times. The 

 temporary character of the effect indicates that it is due 

 rather to the moisture of the breath than to any greasy con- 

 tamination; a view confirmed by subsequent experiments, in 

 wdiich the breath was replaced by a current of pure air which 

 had passed through warm water, 



In the experiment with a powdered surface, the dust may 

 be driven from the neighbourhood of a drop of petroleum by 

 the action of vapour without actual contact of the liquids. 



Drops of Bisulphide of Carbon upon Water. 



The behaviour of a drop of CS 2 placed upon clean water is 

 also at first sight an exception to Marangoni's rule. So far 



