April 24, 1902] 



NA TURE 



589 



saturated water vapour only condenses in presence of 

 suitable nuclei, or the production of double refraction by 

 electrostatic stress (Kerr effect) in liquids, or Faraday's 

 pretty experiments on the electrolytic action of the 

 current from a frictional machine, or the beautifully 

 ingenious demonstration, by manometric measurement of 

 the pressure of an air bubble between the plates of a 

 condenser immersed in an insulating fluid, of the pressure 

 perpendicular to the lines of force in the electric field and 

 its relation to the difference of potential between the 

 plates. This last experiment was originally devised by 

 Quincke in connection with an important research on 

 the dielectric constant (sp. ind. cap.) of liquids, which 

 he measured in three different ways — the ordinary 

 " capacity " method, the measurement of the attraction 

 between the plates in air and in the liquid {i.e. of the 

 force parallel to the field), and, as just mentioned, of the 

 pressure perpendicular thereto. The three methods 

 yielded results which were, on the whole, in vety satis- 

 factory agreement 

 with one another 

 and with Clerk 

 Maxwell's theory, 

 e.g. for CS2 2'64, 

 2'67, 274 (square 

 of refractive index 

 for D line 2-69). 



This was only 

 one of a long se- 

 ries (1880-1888) of 

 electric and mag- 

 netic researches. 

 These dealt, i)iier 

 «//«, with the alter- 

 ation in elasticity, 

 volume and refrac- 

 tive power pro- 

 duced by electrifi- 

 cation (" electric 

 expansion " and 

 electrostatic double 

 refraction, with the 

 suggestion that the 

 latter may be ex- 

 plained by the 

 former). Deserv- 

 ing of especial 

 mention is the 

 discovery of the 

 changes in level 

 of magnetic (and 

 diamagnetic) li- 

 quids contained in 

 the capillary limb 

 of a U-tube placed 

 in the magnetic 

 field. These changes are due to the difference of 

 the "magnetic pressures'' perpendicular to the lines 

 of force in the liquid and the surrounding gas (compare 

 the electric case above) ; they are proportional to the 

 susceptibility of the liquid and the square of the field 

 strength, and if the latter be independently determined, 

 the susceptibility is deduced. I5y varying the gas in 

 contact with the liquid, the susceptibility of gases may be 

 measured. A method similar in principle is applicable 

 to solids in the form of wires or electrolytic deposits on 

 suitable rods. When once the susceptibility of suitable 

 liquids (ferric and manganese chlorides are among the 

 best) has been determined, the method can be used with 

 great advantage to measure the strength of magnetic 

 fields. 



All this opened a wide field of research, but space 

 forbids us to dwell on the interesting developments 

 respecting "atomic magnetism" and other points. 



Quincke is probably most widely known by his re- 

 searches in capillarity, which have been of the most 

 extensive and laborious kind. For the fundamental 

 liquids, as we may call them, i.e. water and mercury, he 

 has made great use of the methods of flat drops, and of 

 air bubbles in the liquid, and therewith found values of 

 the surface tension somewhat higher than those obtained 

 by himself and others in other ways, in particular (for 

 water) by the well-known capillary-tube method. He 

 regards these higher values as the more probable, and 

 concludes that the angle of contact of water and glass is 

 not zero, as usually assumed, and therefore the water 

 does not rise so high in capillary tubes as it would if the 

 angle actually vanished. He has confirmed this by 

 devising methods for measuring this angle of contact 

 both on flat surfaces and in capillary tubes, and thus 

 shown that for water and glass the angle may be 20° or 

 so. He has also proved how sensitive the angle is to 

 slight and often imperceptible changes in the condition 



-^z^e-^^'v^ L^loc^<^u^<>»cc^ 



of the surface. For water on cleavage surfaces of mica, 

 for instance, it varies from o^ to 30' or more, according 

 as the surface is quite fresh or has been exposed for shorter 

 or longer periods to the air. By this principle of the 

 variation of the angle of contact, and consequently of the 

 size and appearance of small drops of liquid (water, 

 mercury) deposited on a surface different parts of which 

 are in imperceptibly different conditions, he has explained 

 the formation of the curious " breath figures " and of 

 Daguerreotype photographs. Daguerre discovered these 

 through the accident of having left some of his silver 

 iodide plates, which he had until then been unable to make 

 permanent, in a cupboard where some mercury had been 

 spilt. The vapour deposited itself in different-sized 

 droplets on the different parts of the plate and gave a 

 picture which could be made permanent. 



The very e.xtensive series of researches Quincke has 

 made on salt solutions lead to the general result that the 



NO. 1695, VOL. 65] 



