1890.] check the Motions of Camphor upon Water. 



3()5 



surface the quantities of grease required may be brought easily 

 within the scope of a sensitive balance. 



In the present experiments the only grease tried is olive oil. It is 

 desirable that the material which is to be spread out into so thin a 

 film should be insoluble, involatile, and not readily oxidised, require- 

 ments which greatly limit the choice. 



Passing over some preliminary trials, I will now describe the 

 procedure by which the density of the oil film necessary for the 

 purpose was determined. The water was contained in a sponge-bath 

 of extra size, and was supplied to a small depth by means of an india- 

 rubber pipe in connexion with the tap. The diameter of the circular 

 surface thus obtained was 84 cm. (33"). A short length of fine 

 platinum wire, conveniently shaped, held the oil. After each opera- 

 tion it was cleaned by heating to redness, and counterpoised in the 

 balance. A small quantity of oil was then communicated, and deter- 

 mined by the difference of readings. Two releasements of the beam 

 were tried in each condition of the wire, and the deduced weights of 

 oil appeared usually to be accurate to milligram at least. When 

 all is ready, camphor scrapings are deposited upon the water at 

 two or three places widely removed from one another, and enter at 

 once into vigorous movement. At this stage the oiled extremity of 

 the wire is brought cautiously down so as to touch the water. The 

 oil film advances rapidly across the surface, pushing before it any 

 dust or camphor fragments which it may encounter. The surface of 

 the liquid is then brought into contact with all those parts of the 

 wire upon which oil may be present, so as to ensure the thorough 

 removal of the latter. In two or three cases it was verified by trial 

 that the residual oil was incompetent to stop camphor motions upon 

 a surface including only a few square inches. 



The manner in which the results are exhibited will be best explained 

 by giving the details of the calculation for a single case, e.g., the 

 second of December 17. Here : 81 milligram of oil was found to be 

 very nearly enough to stop the movements. The volume of oil in 

 cubic centimetres is deduced by dividing 0*00081 by the sp. gr., 

 viz., 0*9. The surface over which this volume of oil is spread is 



^ 7r x 84 2 sqaare centimetres ; 



so that the thickness of the oil film, calculated as if its density were 

 the same as in more normal states of aggregation, is 



0-00081 1-63 

 . — cm 



0-9xJttx842 ~ 107 



or 1*63 micro-millimetres. Other results, obtained as will be seen at 

 c onsiderable intervals of time, are collected in the Table. For conve- 



