October 1, 1900.] 



KNOWLEDGE. 



235 



in the state of powJor or lihugs, aoid al^w .ai-ious i 

 mixtures of metallic powders with non-conducting ones, 

 which ordinarily offer iui extiemoly high resistance to 

 the passage of an electric current, fell enormously and 

 quit-e suddenly in resistance whenever an electric sp;u'k 

 occurred in the neighboiu-hood. This lowered resistance 

 continued for some time, but the powder could be in- 

 stantly restored to its high resistance state by tapping 

 it, and in some cases by increasing the temperature. 

 Branly found that when the powders had once 

 been submitted to powerful electric action mechanical 

 shocks did not restore them entirely to their 

 original state, but that they continued to show them- 

 selves very much more sensitive to electrical actions. 

 Some few bodies, such as peroxide of lead, had their 

 lesistance increased by the action of the electric sparks, 

 and others again had their resistance alternately in- 

 creased and diminished. The last results ai'c curious 

 and interesting, but the important case for its applica^ 

 tion to Hertzian telegraphy is that of diminished 

 resistance. Branly's results became known to Professor 

 Lodge at the end of 1893, when he at once proceeded 

 to try the Branly tubes of filings, and found them greatly 

 superior in manageabUity to either the Boltzmanu gap 

 or his own delicately adjusted cohering knobs, but imme- 

 diately afterwards he, in conjunction with Professor 

 Fitzgerald, devised a coherer consisting of a sewing 

 needle resting upon aluminium foil, which they found 

 to be of extraordinai-y sensitiveness and at the same time 

 reasonably manageable. Professor Lodge then made a 

 whole series of what he describes as quasi-optical 

 experiments with the new detector, and, before long, 

 various improved methods of airanging the filings were 

 discovered, especially that of sealing them up tn vacuo 

 or in hydrogen, in order to protect them from oxidation 

 by the air, the effect of which would be to produce too 

 great a thickening of the extremely thin film separating 

 them from one another. When brass filings immersed 

 in hydrogen were used, they soon became too clean, and 

 their sensitiveness so great that it was impossible to 

 restore the original high resistance by tapping. Pro- 

 fessor Lodge consequently preferred the vacuum ob- 

 tained by the use of a Spreugel mercury pump. He 

 states that almost any filings tube was capable of detect- 

 ing signals sent from a distance of 60 yards, with a mere 

 six-inch sphere used as oscillator, and without the 

 slightest trouble, but that he found the single point 

 coherer much more sensitive than any filings tube. 



For tapping back, the use of an electric beU mounted 

 on the base of a filings tube was not found very 

 satisfactory, owing to the disturbances produced by the 

 small sparks occurring at its contact breaker, to which 

 this more delicate detector responded as well as to the 

 signals which it was meant to attend to, while the less 

 delicate knob apparatus had not been so affected. A 

 tapper consisting of a rotating spoke wheel driven by 

 the clockwork of a Morse instrument and giving the 

 coherer a series of jerks at regular intervals was there- 

 fore employed. 



Mr. Rolls Appleyard and Lord Rayleigh have devised 

 a liquid coherer consisting of two globules of mercury 

 separated by a thin film of grease, such as paraffine 

 oil. When a battery cell is connected up in a circuit 

 with these globules, they are pressed together evei-y time 

 the circuit is closed, and Lord Rayleigh has observed 

 that it takes an appreciable time before they come into 

 contact, aa though a film had to be mechanically squeezed 

 out from between the oppositely charged metallic sur- 



faces, and this suggests that cohesion may in every case 

 be simply a resvdt of electrostatic attraction, and that the 

 moleculai" films separating solids in contact may be 

 squeezed out in a similar manner. The force of attraction 

 between two surfaces difl'eriug in jjotential by a volt 

 and separated by the smallest known thickness of thin 

 film (which is about 10.7 centimetres) would be equiva- 

 lent to about 650 pounds to the square inch, a quite 

 sufficient pressure to make this explanation a perfectly 



possible one. 



♦ 



PLANTS AND THEIR FOOD. V. 



By H. H. W. Peaeson, m.a. 



We have now to consider the means by which the food 

 constituents of the soil enter the plant and are cairicd 

 ujjwards to the leaves ; for it is in the cells of the leaves 

 that the food supplies from the atmosphere and from the 

 soil axe brought together and luidergo chemical changes. 

 This important work of the absorption of mineral food 

 from the soil is entrusted to the root, which in most 

 cases serves the additional pui-jsose of holding the plant 

 firmly in fiosition, and frequently also acts as a store- 

 house in which is laid by food or water for future use. 



if a bean or pea be soaked in water until the embryo 

 swells and bursts the seed-coat, it is seen to consist of 

 two comparatively large and thick embi-yo-leaves or 

 cotyledons, a minute bud or " plumule " between them, 

 and, in a straight line with this, projecting beyond the 

 cotyledons, a very small papilla, which is the embryonic 

 root or " radicle." The cotyledons contain so much 

 organic food matei-ial — starch and proteid — that the 

 embryo in the early stages of its growth needs no root, 

 but grows at the expense of these materials stored up 

 for its use by the mother-plant. Meanwhile the tiny 

 radicle insinuates itself between the soil-pai-ticles and 

 grows downwards, and later gives off minor branches, 

 and so becomes capable of laying the soQ under con- 

 tribution to supply the mineral needs of the common- 

 wealth of which it forms a part, as soon as the stores 

 in the cotyledons are exhausted. In many plants — e.g., 

 the Grasses — the radicle perishes almost as soon as it 

 emerges from the " seed,' and is replaced by numerous 

 branches arising from the scar left by the defunct radicle 

 or from the lower part of the stem or leaves. 



The fact that roots grow downwards is so well known 

 that mention of it may seem superfluous. This habit is 

 due to the influence of gi-avity. It is easily noticed 

 that the main root (produced by the continued growth 

 of the radicle) is more strongly influenced by the force 

 of gravity than are its side-branches, for while it strikes 

 a course which is, in the main, towards the earth's 

 centre, its branches make a considerable angle with it, 

 and frequently grow in a horizontal direction. Con- 

 sequently the roots are able to exploit a much larger 

 area of soil than would be possible if the branches 

 and the main root were equally amenable to gravity. 

 A fui-ther peculiarity in the growth of most roots is 

 that they shun the light and take the shortest course 

 to dark or shady places. On an ivy stem the clinging 

 roots by which it is attached to the wall or tree all 

 emerge on the shady side of the stem and proceed at 

 once to bury their sensitive tips in the nearest hollows 

 of the support. There arc, on the other hand, roots 

 which behave like stems, in that they bid defiance to 

 gravity and gi'ow erect and show no tendency to hide 

 themselves from the light. These, however, are quite 

 exceptional, and as a rule serve the planta which pro- 



