March 17, 19 10] 



NATURE 



A Radium Experiment. 



It is usual to demonstrate the ionising property of 

 radium by discharging an electroscope. The reverse ex- 

 periment of charging an electroscope, or Leyden jar, is 

 more effective, and can be made to ring a bell. 



If a wire, about 6 inches in length, is coated with a 

 salt of radium and placed in contact with the knob of an 

 electroscope, the latter will quickly become charged if any 

 charged body is placed within 2 or 3 feet. A small 

 Wimshurst machine is very convenient for the demonstra- 

 tion, as it can be easily turned about to reverse the charge, 

 or placed equatorially, when no efifect is produced. 



It is a good plan to arrange the leaf of the electroscope 

 to discharge itself automatically to earth at a divergence 

 of about 45 degrees ; then the charging is repeated so long 

 as a charged body is in the vicinity. Instead of sending 

 the discharge to earth it may pass into a coherer, and so 

 ring a bell or give a record. 



When an induction coil is used it is easy to show that 

 the prepared electroscope will keep on receiving a charge 

 for some minutes after j^e coil is stopped; for instance, 

 after worlcing an iS-inc^fCoil for one minute, some charge 

 was given to a delicate radium-tipped electroscope when 

 brought into the room four minutes later. 



This duration of charge on air can be distinctly shown 

 by filling a large paper bag with charged air and rapidly 

 conveying the same to a Wilson electroscope in another 

 room. 



This is obviously due to a residual charge in the air, 

 which persists long after re-combination of mixed ions. 

 This arrangement of a radio-active body near, or, better, 

 in actual contact with, a delicate electroscope is a very 

 sensitive detector. The effect is not quite so good if the 

 radio-active coated wire is placed in contact with the 

 charged body, leaving the electroscope free. Upon ex- 

 ploring the charged air of a room, it is usually found to 

 be divided into two areas of opposite charge ; these can be 

 mixed by waving an umbrella, so yielding a neutral 

 mixture. 



The fact that a charge is so easily acquired by an 

 electroscope must be taken into consideration during 

 delicate testing, for the mere act of withdrawing a 

 vulcanite pen from the pocket will give a negative charge 

 to a distant electroscope in ionised air. 



F. Harrison Glew. 



156 Clapham Road, London, February 5. 



SUBSTITUTES FOR RUBBER. 



n^HE present demand for india-rubber naturally 

 J- directs attention to those articles which, to a 

 greater or less degree, may serve to replace rubber in 

 its industrial applications, and so help in conserving 

 the supply. 



Of such articles a very large number have been 

 proposed. Those in actual use to any considerable 

 extent are, however, relatively few. For present pur- 

 poses the various surrogates may be distinguished as 

 (i) rubber-substitutes proper, consisting wholly of 

 ingredients other than rubber ; (2) composite or 

 "artificial" rubbers, which contain a certain propor- 

 tion of natural rubber worked up with other sub- 

 stances ; and (3) true synthetic rubber, namely, a pro- 

 duct containing the rubber molecule synthesised in the 

 laboratory or factory by chemical means from simpler 

 compounds. 



At present the first of these classes is commercially 

 the most important. Scores of recipes are in exist- 

 ence, including very diverse ingredients ; but the basis 

 of most is a modified oil. At first sight there seems 

 little suggestion of india-rubber in the properties of an 

 ordinary- vegetable oil, but a simple experiment will 

 indicate the kind of modification which certain oils 

 readily undergo, and which help to fit them for use 

 as rubber substitutes. If we test the drying proper- 

 ties of boiled linseed oil by spreading a little of it 

 over a slip of glass and allowing it to dry, a film of 

 oxidised oil is eventually obtained, having a certain 



NO. 2107, VOL. 83] 



modicum of toughness and elasticity. The liquid oil 

 has taken up oxygen, and thereby become converted 

 into a more or less elastic solid. Tung-oil suDstitute is 

 essentially such an oxidised product, manufactured by 

 heating the raw oil until it has absorbed enough 

 oxygen to cause it to thicken and become solid on 

 cooling, when it is powdered and worked up with a 

 little petroleum. 



In a somewhat similar way the oils can be made to 

 take up sulphur, becoming thereby solid, and en- 

 dowed in some degree with elastic properties. The 

 treatment is analogous to the " vulcanisation " of 

 rubber. "Brown" or "black" substitutes are manu- 

 factured by heating the oil with sulphur, a process 

 corresponding to the " hot cure " method of vulcanisa- 

 tion. "White" substitutes may be made bv merely 

 mixing the oil, cold, with 20 to 40 per cent, of sulphur 

 chloride; or, better, by first dissolving the oil in a 

 suitable solvent such as carbon tetrachloride. This 

 resembles the "cold cure" process used in vulcanising 

 rubber. Colza oil is largely used for these purposes, 

 but various others are available — linseea, maize, 

 arachis, and castor oils, for example. The chemical 

 reaction involved is a somewhat compjicated one, but 

 probably it consists mainly in the formation of what 

 chemists term an "addition-product." The proportion 

 of sulphur taken up by the substitutes varies rather 

 widely, ranging from 5 to upwards of 15 per cent. 

 .As would be expected, oils which have previously been 

 oxidised to a notable extent (e.g. " blown " oils) re- 

 quire less sulphur to saturate them than do the natural 

 oils. 



" Nitrated " oils are also used as the basis of some 

 rubber surrogates. Thus one well-known product is 

 a solution of a nitro-cellulose in linseed or castor oil 

 which has been nitrated by treatment with a mixture 

 of nitric and sulphuric acids. Other such articles are 

 made by oxidising the nitrated oil with lead peroxide, 

 or by simply heating it in air. 



These oxidised, sulphured, and nitrated oils, in one 

 form or another, are largely used as substitutes for 

 rubber. Of the other substitutes proposed, a few 

 examples may be given, to indicate something of their 

 general nature. 



First there are those which, while still retaining 

 oil as one ingredient, include also other important 

 constituents. Thus, "Fenton's rubber" is a mixture 

 of oils with tar, pitch, and creosote; which mixture, 

 when digested with nitric acid, gives a toughened 

 mass, and this on heating yields an elastic product 

 simulating rubber. " Russian " substitute, said to be 

 useful for covering telegraph cables, contains as in- 

 gredients wood-tar, hemp and linseed oils, ozokerite, 

 spermaceti, and sulphur. " Oxolin " is made by 

 impregnating fibrous material such as jute or hemp 

 with linseed oil, oxidising the oily mass with warm 

 air, and working the product up between rollers into 

 a coherent mass, which can then be vulcanised by 

 heating it with sulphur. 



In another category of substitutes oil plays only a 

 subordinate part, or is altogether dispensed with. Thus 

 "Jones's substitute " is stated to be made from various 

 gums and gum-like products as the chief constituents. 

 In W. H. Perkin's patent (23,031/07), gelatine or glue 

 is dissolved in creosote and then treated with some 

 reagent — potassium bichromate, formaldehyde, or 

 tannic acid — which will render the gelatine or glue 

 insoluble; after "setting," the mass obtained is 

 digested with acetone to make it firmer. "Textiloid" 

 has for its ingredients various resins, nitrocellulose, 

 and camphor. As a curiosity in this class mav be 

 mentioned "grape rubber," produced from the skins 

 of grapes by means of pressure; it is not, however, a 

 commercial article. Finally, though this can only be 



