6io 



NA TURE 



[April 27, 1905 



TANTALUM. 



T^ HE applicalion of electricity to chemical problems 

 ^ has again borne fruit in the isolation and pre- 

 paration of tantalum. Dr. Werner von Bolton, of 

 the firm of Siemens and Halske, published the results 

 of his very interesting' research upon the preparation 

 of tantalum in the Zeitschrift fiir Elektrochemic 

 (January 20). Although the existence of tantalum 

 was pointed out by Hatchett in 1801, it does not 

 appear up to the present to have been prepared in 

 the pure condition. Moissan, indeed, in 1902 pre- 

 pared the metal by reducing tantalic oxide (Ta.,0,) 

 in the electric furnace. But the metal was extreriiely 

 hard and brittle, a property which Dr. Bolton now- 

 shows only belongs to the impure product; Moissan 's 

 metal probably contained some carbide. Dr. Bolton 

 has succeeded in obtaining the metal by an electrical 

 and b)- a chemical method. 



The Electrolytic Method. 



As is well known, Ncrnst found that when a thin 

 rod of magnesia (MgO) is heated to whiteness it 

 becomes able to conduct the electric current, the 

 magnesia being split up into its components, mag- 

 nesium and oxygen; the magnesium, however, 

 immediately re-combines with oxygen, the process of 

 electrolysis therefore becoming continuous. Other 

 metallic oxides, such as zirconium, vtterbium, 

 thorium, calcium, and aluminium, &c., likewise be- 

 have in a similar manner. If, now-, a rod of mag- 

 nesium is strongly heated in vacuum and the electric 

 current passed through it, the o.xygen given off is so 

 dilute that re-combination does not take place, and 

 the rod becomes powdered. Dr. Bolton, working 

 along somewhat similar lines, found that the coloured 

 or low-er oxides of vanadium, niobium (columbium), 

 and tantalum will conduct the electric current w-ith- 

 out the necessity of being heated to very high 

 temperatures. Strange to sav, the colourless or 

 higher oxides have not this property. 



In order to prepare tantalum in this manner a 

 filament of the brown tantalum tetroxide (Ta.,0,) w-as 

 prepared and fixed into an evacuated globe, which 

 was connected w-ith a vacuum pump, so that if oxygen 

 was given off, on heating, it could be pumped' out. 

 On passing a current through this filament, at first 

 the two ends of the filament became white hot, and 

 then gradually the incandescence travelled along the 

 filament until the w-hole of it became incandescent. 

 A large quantity of oxygen w-as given out, and the 

 filament, which at the commencement was brown, 

 became metallic grey. The tantalum so obtained 

 showed on analysis a purity of 90 per cent. 



The Chemical Method. 



Details as to how the chemical method is carried 

 out are not given. Dr. Bolton simpiv savs that the 

 metal can be obtained by fusing a mixture of potas- 

 sium tantalum fluoride w-ith potassium by means of 

 the electric arc furnace in a vacuum. This method 

 IS a modification of that used by Berzelius in 1824. 



Properties of the Metal. 

 One of the most remarkable properties of the metal 

 l^ its extreme ductility combined with extraordinary 

 hardness. The red-hot metal can readilv be rolled 

 into sheets and foil, and easily drawn' into wire. 

 When the sheet is again heatc'd and hammered it 

 becomes so extremely hard that il was found im- 

 possible, by means of a diamond drill, to bore a hole 

 through a sheet i mm. thick. The drill, rotating 

 5000 limes to the minute, was worked day and night 

 NO. 1852, VOL. 71] 



for three days, and at the end of the time had only 

 made a depression 0.25 mm. deep, while the diamond 

 of the drill was very much worn. This property may 

 very probably lead to its being used for drills in pface 

 of the diamond. 



The metal melts between 2250° and 2300°. The 

 atomic heat agrees with the law of Dulong and Petit, 

 being 6.64. The specific gravity is 14.08. When two 

 electrodes of tantalum are placed in a bath of dilute 

 sulphuric acid, the tantalum becomes passive, and 

 t even with an E.M.F. of 220 volts at the terminals 

 no current passes. When placed opposite an electrode 

 of platinum only one phase of an alternating current 

 passes ; it may thus be used for rectifying an alter- 

 nating current in the same manner that aluminium 

 can. 



In the form of wire, sheet or ingots, the metal is 

 unacted upon by sulphuric, hydrochloric, or nitric 

 acid, and even by aqua regia. 

 Hydrofluoric acid reacts very 

 slowly, unless the metal is in 

 contact with platinum, for 

 example, in a platinum dish, 

 when it dissolves readily with 

 evolution of hydrogen. Fused 

 alkalis have no action upon it. 



When made the kathode in 

 an acid electrolyte it absorbs 

 hydrogen, which is only par- 

 tially given up, even when 

 the metal is fused. The metal 

 may be lieated to red heat in 

 the air w-ithout taking fire. 

 At 400° it turns slightly 

 yellow, at a low red heat it 

 turns blue, and finally be- 

 comes coated with a w-hite 

 protective coating of the pent- 

 oxide. It absorbs nitrogen at 

 a white heat, and unites with 

 sulphur when melted with it 

 under fused potassium 

 chloride. Tantalum appar- 

 ently forms no amalgam with mercury, although it 

 produces alloys with most other metals. When united 

 w-ith I per cent, of carbon it becomes hard and brittle, 

 and can no longer be drawn into w-ire. 



.As alreadv stated, the original idea in working with 

 tantalum was to find a new material to be used for 

 incandescent electric lamps. The first experiments 

 were tried w-ith the oxides of vanadium and niobium 

 (columbium) ; the coloured or lower oxides of these 

 metals w-ere found to conduct the current and to give 

 up their oxygen w-hen thus heated in vacuum. 

 Vanadium so obtained was found to melt at 1680° 

 and niobium at 1950° ; but owing to these compara- 

 tively low- melting points they could not satisfactorily 

 be employed for electric lighting purposes. Tantalum, 

 however, which melts between 2250° and 2300°, has 

 been successfully employed for this purpose by Messrs. 

 .Siemens and Halske. 



Filaments of the metallic tantalum are fused into 

 a globe, which is then evacuated in the usual manner. 

 The first lamp was made with the usual bow-shaped 

 filament, and required 0-58 ampere w-ith a pressure 

 of 9 volts, giving 3 candle-power. It was then found 

 that in order to produce a 22 candle-power lamp suit- 

 able to being placed on a i lo-volt circuit more than 

 20 inches length of filament w;is required. The diffi- 

 culty presented was to get this great length of fila- 

 ment convenientlv into the ordinary sized globe. The 

 illustration (taken from the Electrical Magazine for 

 .March) shows how- the dilViculty was got over. The 

 central support is a rod of glass, having a number of 

 wires radiating from it to act as supports. This 



