250 Mr. C. Tomlinson on the Cohesion-Figures of Liquids. 



one or other liquid : where there is no solution, there may be 

 simply adhesion. In both cases, whether there be solution or 

 not, one of the liquids displays the phenomena of cohesion in a 

 characteristic manner. For example, the essential oils are but 

 slightly soluble in water. If we place a drop of oil of lavender 

 on the surface of water, the adhesion of the water will cause it 

 to spread out into a film j but the cohesion of the oil immediately 

 begins to reassert itself; the film opens in a number of places, 

 forming long irregular arms or processes resembling the pattern 

 assumed by wood when it has been much worm-eaten. These 

 processes tend to gather up into separate discs or lenticules ; the 

 adhesion of the water spreads them out, the cohesion of the oil 

 struggles to prevent this, and soon prevails; the almost immediate 

 issue being the formation of the original drop into a number of 

 discs with sharp, well-defined outlines and convex surfaces. The 

 action is often so rapid, and the pattern so complicated, that it 

 requires repeated observation to become master of all the phe- 

 nomena. (See Plate IV. principal figure, and the subsidiary 

 figures a, b, c, d.) 



Now this struggle on the part of the oil of lavender to pre- 

 serve its cohesion gives rise to a figure which is characteristic of 

 the substance, and which I propose to name its cohesion-figure. 

 It may be regarded as the resultant of the cohesive force of the 

 substance, its density, and the adhesion of the surface on which 

 it is placed. I believe that every independent liquid has its own 

 cohesion-figure. By an independent liquid, I mean not a solu- 

 tion ; for in the solutions of solids and liquids cohesion has been 

 already overcome. 



The cohesion-figures of liquids can be conveniently studied by 

 gently placing on the surface of water, of mercury, &c. a drop 

 of the substance in question, which we will suppose exerts no 

 chemical action on the receiving surface. Now the cohesion- 

 figures of liquids will be more or less permanent in the inverse 

 ratio of the solubility of the substance. A drop of one of the 

 fixed oils placed on the surface of water will spread out into a 

 film, which is characteristic of the substance, and may last some 

 minutes or even hours, according to the degree of force with 

 which cohesion reasserts itself. A drop of one of the essential 

 oils will also give a characteristic film or cohesion-figure which 

 may change every moment from evaporation and display some 

 beautiful effects of colour ; but all these phenomena will be cha- 

 racteristic of the substance in question, and will enable it to 

 be recognized. A drop of a substance like creosote, which is 

 slightly soluble in water, may continue five minutes ; a drop of 

 ether or of alcohol may last only a fraction of a second; but 

 whether the time be long or short, these figures are typical of the 



