620 



NATURE 



[July 14, 1921 



currents, and remain only as short lengths of particles 

 held together. To throw light on this possibility, a 

 small vertical, hollow cylinder of plaster of Paris 

 open above was arranged with iron electrodes (for 

 forming an arc) passing through its sides and meet- 

 ing in the centre. Bv passing the current of a storage 

 battery giving about 50 volts through them in con- 

 tact and separating them, an iron arc could be pro- 

 duced at will within the plaster cylinder. The dimen- 

 siois of the cylinder were such that a microscope 

 slide 3 in. by i in. could rest across the open upper 

 end of the plaster cylinder, only partly closing it, the 

 slide lying horizontally above the arc electrodes at a 

 distance of about 3 cm. Such a slide could receive 

 a layer of smoke on its under-surface when the arc 

 was formed below it. The microscope in that case 

 showed only a confused deposit. 



When, however, there was placed above the slide 

 a strongly excited electromagnet with its poles rest- 

 ing on the upper sides of the slide or close thereto, 

 such poles being about 3 cm. apart, a smoke deposit 

 of a remarkable character was produced. Even as 

 examined by the unaided eye in diffused light there 

 was decided evidence of a structure or striation. 

 When the microscope was used, with even com- 

 paratively low powers of about 300^400 diameters, 

 there was disclosed a decided striation seemingly 

 composed of brownish particles in strings extending 

 over the slide and following the direction of the 

 field. There was noted a surprising regularity in 

 the distribution or spacing of the striae, as if the 

 surface was covered with fibres laid on systematically 

 side by side. 



There were, however, curious objects composed of 

 small spheres (evidently globules of iron) strung 

 together in a line of two, three, four, or more, such 

 spheres having no uniform size. Most of these iron 

 globule groups la)', of course, in the field direction, 

 and were very large relatively to the particles in the 

 striation covering most of the surface of the slide. 

 But each of these straight settings of globules pos- 

 sessed a singular appendage, generally at one end 

 only, but sometimes at both ends. It consisted of 

 a brush-like tail composed of the brown filamentous 

 chains of particles like those covering the slide as 

 noted above. They gave the appearance of tufts, 

 suggesting a growth of fine beaded fibres from the 

 end of the string of globules. By focussing, these 

 tufts or tails could be seen as projecting outward 

 (upward) in an inclined direction. This means that 

 the tufts did not lie on the slide surface, but sprang 

 outward from the globule which carried it. The 

 globule at the other end of the short chain (generally 

 the largest in the line) was often to be seen as having 

 a convergence upon it of the usually parallel striae of 

 the other parts of the slide, indicating clearly that the 

 globules strung together were acting as small magnets 

 with poles at each end, towards and from which poles 

 the convergence and divergence of the magnetic lines 

 were indicated by the fine striae of particles taking 

 their direction. 



The pyolariscope showed that the striated smoke 

 layer caught on the slide has the same property of 

 scattering or diffusing light (as plane polarised light) 

 that the smoke oriented in the air by a magnetic field 

 has, but, of course, the slide preserves the orientation, 

 and needs, to produce the results, no magnetic field 

 after its formation or •deposition. The slide covered 

 with the striated smoke film is, in fact, a polariser. 



Examination between crossed Nicol prisms (dark 

 field) discloses the fact that the tufts of fine fibres 

 carried by the rows of globules show as luminous 

 spots on the black field, clearly indicating that the 

 groups or tufts have a polarising effect if they are 

 in proper relation to the rays passing through. 



NO. 2698, VOL. 107] 



As was to be expected, any hollow vessel or en- 

 closure capable of retaining the smoke from an iron 

 arc can be used in demonstrating the original 

 luminous phenomenon. A glass flask of from i to 

 2 litres is readily sensitised, as it were, by holding 

 its mouth over an iron arc for a short time, allowing 

 smoke from the arc to enter, and then corking the 

 flask.^ It may then be used to show the effects by 

 allowing a beam of light to traverse it while held in 

 the field of a current-carrying coil. While this was 

 being done it was noticed by Dr. Hollnagel (of the 

 laboratory) that when the coil was traversed by an 

 alternating current of twenty cycles the flask, when 

 near the coil, gave the usual effect of increased 

 luminosity of the smoke in its interior. When, how- 

 ever, the flask was removed from the coil a distance 

 of several feet the steady luminosity was replaced by 

 a flickering which kept pace, not with the alterna- 

 tions of current in the coil, but with the cycles only. 

 The flickering was, as it appeared, at the cyclic 

 rate. This flickering was noted even at a distance 

 of 12 ft. from the coil, although the coil was but 

 7 in. in diameter and about 2 in. in axial direction. 

 The flickering is a curious effect, and it is diflicult to 

 explain, especially the fact that it appears to keep 

 time with the cycles, and not with the alternations, of 

 current. It points to some sort of magnetic retention 

 or polarisation of the iron particles of the smoke. 

 They may even rotate or oscillate in obedience to the 

 field fluctuations, but there is needed much more 

 work of investigation as to the cause of the peculiar 

 behaviour. The experiment clearly shows that a very 

 moderate field intensity suffices for lining up the 

 particles in the air, and so producing the luminous 

 effect. 



Emphasis is again given to the fact of the extremely 

 small amount of iron particles suspended in the air. 

 capable of giving a decided effect. 



Elihu Thomson. 



Thomson Laboratory, Lynn, Mass., June 17. 



The Japanese Artificially Induced Pearl. 



The subject of artificial pearl induction, I venture to 

 suggest, affords an excellent example of comparative 

 pathology. Dr. Lyster Jameson's diagram in Nature 

 of May 26, p. 396, might well pass as an illustration 

 of "pearls" frequently found in the human body. 

 Such "pearls" are commonly seen In papillomata 

 of the skin and at muco-cutaneous areas, but they can 

 also be demonstrated in the tonsils, brain-coverings, 

 thymus and thyroid glands, etc. Those which are epi- 

 dermal become keratinoid, but others of deeper origin 

 are often calcified. 



All "pearls," whether ostreal or human, start in 

 columnar cells and undergo metaplastic changes. 

 Those of a wart become horny ; those of the oyster 

 calcified. The histological changes in the oyster are 

 simply a matter of degree, and not difference. 



The diagram fully illustrates this. The "blister" 

 if seen in horizontal (transverse) section would present 

 the same features as seen in the "pearl" — a con- 

 centrically laminated core surrounded by a single layer 

 of cubical cells, embedded in mesoblast if growing, 

 but when growth stops the cubic^.l layer would be no 

 longer seen. 



"Islands" or "rests" of epithelial- elements are 

 common in man. In the oyster such an inclusion may 

 become the true pearl and grow like a wart. Artificial 

 induction or grafting merely imitates the natural pro- 

 cess, and its later history is simply a matter of slight 

 change In degree. In either case the pearl rnust be 

 viewed as a morbid structure due to focal irritation. 

 It is held that a wart may become malignant. In 



