August, igo6.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



503 



After seemingly remaining unaltered for 5-10 minutes, 

 these continue their evolution. In fact, tlie carbon 

 particles contained in the Chinese ink, and which at 

 first, owing to diffusion, had left the centre of each 

 of the drops, will go back again in the original direc- 

 tion, tending to combine in the centre of the drops, 



■ ^.^V'^V^-vv^;^ 



•**> 



Fig. 2. 



while small groups of particles accumulating at the 

 same time in the lines of direction will form ever- 

 increasing grains (Fig. 2), which travel towards the 

 centre of the drops (Fig. 3). If evolution be allowed 

 to continue, the Chinese ink will thus be found to com- 

 bine in a liquid hectagonal ring, and finally in a homo- 



.*i^ 



r 



gLMicous mass. .'\ similar drop, as rupresented in Fig. 4, 

 in natural size, had the dimensions of an ordinary drop 

 (of no more than 3 nun. diameter) on being introduceil 

 into the solution, but afterwards, owing to diffusion, 

 reached a diameter considerably higher than that of 

 the photograph, thus assuming the shape of a negative 

 field of diffusion. The drop is afterwards retracted into 



itself, its centre becoming darker and darker, while iti 

 lines of force are separated into ever-increasing grains 

 approaching towards each other and towards the 

 centre of the drop, until they take the aspect repre- 

 sented in Fig. 4. 



While not Ix-ing explained by the action of diffusion, 

 these phenomena are fully accounted for on that of 

 cohesion. In a liquid like the present potassium 

 nitrate solution, containing a microscopical powder 

 such as the carbon particles of the Chinese ink in sus- 

 pension, the following forces of attraction should, in 

 fact, be considered : (i) TTie cohesion between the mole- 

 cules of the solution; (2) that between the particles 

 in suspension, and (3) the mutual attraction between 

 the floating particles and the molecules of solution. 

 An equilibrium will be produced if these forces be strictly 

 equal to each other, which will only exceptionally 

 be the case. Whenever the forces of attraction are, 

 howe\er, unequal, no equilibrium will be obtained 

 unless there is a perfect symmetry round each of the 

 molecules of solution and each floating particle. So 

 far from being stable, such an equilibrium will Ije dis- 

 continued whenever the perfect symmetry cea.ses. If, 

 ('..^., the attraction between the floating particles be 

 considerably stronger than that acting between these 



and the particles of solution, any accumulation resulting 

 in a disturbance of symmetry will constitute a centre 

 of attraction for other particles, which, by joining the 

 firrmer, will increase their force of attraction. The 

 part incumbent on diffusion now is a double one; it 

 first counteracts the realisation of symmetry in the 

 liquid, setting solid particles and liquid molecules in 

 motion; on the other hand, it carries them into the 

 1 ange of centres of attraction where they come under 

 the sway of cohesion. This is evidently what occurs 

 in the present experiments. 



Professor Leduc has, moreover, been able imme- 

 diately to demonstrate this explanation experimentally 

 by preparing a solution of potassium nitrate coloured 

 with Chinese ink of so precisely the same osmoti- 

 cal pressure as a transparent solution of the same 

 substance, that the coloured drops not penetrating by 

 diffusion into the transparent solution did not undergo 

 ;iny alteration. If, however, the same drops were in- 

 troduced close to each other, they were found to alter 

 tlielr shape and to be lengthened towards each other, 

 approaching one another, and eventually combining 

 into a sphere. \\'henever drops of different dimensions 

 are introduced at equal distances (Fig. 5), the largest 

 drops, exerting a prevalent attraction, will incorporate 

 the small ones. Leduc thus observed (Fig. 5) six small 

 drops arranged around a big one, which, after alter- 

 ing their shape, began travelling towards the larger 



