May 6, 1909] 



NA TURE 



291 



devising methods for producing the rare metals in a com- 

 mercially possible manner, and then to try one after the 

 other as filaments of incandescent lamps. 



While working on these lines Dr. von Bolton succeeded, 

 in the first instance, in producing a vanadium filament 

 by heating a mixture of vanadium pentoxide and paraftin 

 to 1700° C, and thereby producing sticks of vanadium 

 trioxide, which in their turn were heated by electric 

 currents in a glass bulb exhausted by an air-pump, and so 

 converted into metallic filaments. As it was found that 

 vanadium melts at about 1680° C, such filaments were no 

 improvement on carbon filaments, and the next substance 

 to be investigated was niobium, which belongs to the same 

 group of elements, but has nearly double the atomic 

 weight. Treated in a similar manner, the niobium fila- 

 ment gave somewhat better results, but still its melting 

 point, estimated at 1950° C, was too low for practical 

 purposes. 



In this connection it should not be forgotten that at a 

 temoerature considerably below their melting points all 

 these metals begin either to soften or to disintegrate, so 

 that their " working " temperature is not identical with 

 their melting temperature. 



Turning his attention to tantalum, which has an atomic 

 weight of 181, Dr. von Bolton experimented with the 

 black metallic powder produced by the method of Berzelius 

 and Rose, and found that it could be rolled into a fairly 

 coherent mass in the form of ribbons. .Alternative experi- 

 ments, conducted on the lines by which vanadium and 

 niobium had been obtained, resulted in the production of 

 pure tantalum in the form of a metallic button, which 

 was found to be tough and malleable like steel. 



These and other qualities convinced Dr. von Bolton that 

 nobody before him had handled pure tantalum, although 

 Berzelius had first obtained the metal by a chemical 

 process in 1824, and later Moissan succeeded, in 1902, in 

 producing it in his electric furnace. The latter describes 

 tantalum as a hard, brittle metal of the specific gravity 

 of I2-S, and a non-conductor of electricity, but he adds 

 that the substance obtained by him contained about half 

 a per cent, of carbon. 



Considering the high atomic weight of tantalum, this 

 admixture of carbon evidently exercises a great influence 

 on the physical qualities of tantalum, and explains the 

 differences between the observations of Dr. von Bolton and 

 those of his predecessors. In nature, ores containing 

 tantalum are found in many places, principally in 

 Scandinavia. North .America, South-west .\frica, and 

 Western .Australia. Columbite from South Dakota con- 

 tains from 10 per cent, to 40 per cent, of tantalum pent- 

 oxide (Ta,0.,), and a good deal of niobium, combined 

 with iron and manganese in various proportions. 



.As the separation of tantalum and niobiiun is somewhat 

 troublesome, it is preferable to utilise tho tantalite, which 

 consists almost entirely of iron and manganese combined 

 with tantalum pentoxide. From these ores tantalum is 

 separated in the form of a fluoride in combination with 

 potassium (K,TaFl.), and subsequently reduced by metallic 

 potassium to the black powder already mentioned, which, 

 however, still contains some oxide and some hydrogen. 



In order further to purify the product, the powder is 

 pressed into the form of small cylinders, which are melted 

 in a vacuum, by an electric current under certain precau- 

 tion.s, into small buttons of pure tantalum such as are 

 exhibited. 



Since the production of tantalum has been carried out 

 on a commercial scale it has been possible to improve many 

 details of the process, so that the tantalum produced by 

 it at the present time is even purer than that shown in 

 1905 at the discourse of Dr. von Bolton and Dr. 

 Fcuerlein. , 



Some specimens of this latest tantalum have been sub- 

 mitted to Sir James Dewar, who has very kindly made 

 experiments with reference to its specific heat and to its 

 therm.-jl conductivity. He ascertained the specific heat by 

 plunging small spheres of tantalum, which had been 

 heated to the temperature of boiling water, into water of 

 14° C, then transferring them to melting carbonic acid 

 ( — 78° C), and finally to liquid air (—183° C), and as an 

 average of several experiments the specific heat was found 

 to be between 100° C. and 14° C. = oo33, 14° C. and 



NO. 2062, VOL. So] 



-78° C. = oo32, -78° C. and -183° C. = oo28, while Dr. - 

 von Bolton in 1905 gave the specific heat as 00363. . 

 Multiplying these results by the atomic weight (181), it, 

 will be seen that Dr. von Bolton's value (657) is slightly 

 higher and Sir James Dewar's value (5-97) lower thaa 

 6-4, which, according to Dulong and Petit, is the atomic 

 specific heat. 



The result of Sir James Dewar's experiments proves- 

 tantalum to have about three-quarters the conductivity of 

 iron and about one-eighth the conductivity of copper. At_ 

 ordinary temperatures, say below 300° C, pure tantalum 

 resists the action of all acids except fluoric acid, of all 

 alkalies, and of moisture, so that it is an ideal material, 

 for chemical apparatus which do not require high tempera- 

 tures, and for any implements which, when made of steel, 

 are liable to rust. 



It has already been stated that pure tantalum is tough 

 and malleable, so that it can be hammered out into thin 

 sheets or drawn into fine wire, the diameter of the fila- 

 ment wire being 003 mm., or about one eight-hundredth of 

 an inch ; all the same, it is elastic and as hard as soft. 

 steel, and has a tensile strength of 93 kg. per square mm., 

 which is equal to 57 tons per square inch. This means 

 that the filament wire is capable of supporting about 

 80 grams, or 28 ozs., as can be shown by actual experi- 

 ment. 



Tantalum sheet can be stamped into various shapes, 

 and out of bars of tantalum springs can be bent. Another 

 use made of tantalum is as rnaterial for writing pens, 

 manufactured in the usual way. When it was first offered' 

 for this purpose it was found that the material could not 

 pass the test prescribed for pens made of steel. Thes& 

 are pressed by a weight of 180 grams on writing paper 

 which is moving at the same speed as ordinary writing, 

 and while 10 km. {6j miles) of paper are passing the loss 

 by abrasion must not exceed 07 mg. (o-oi grain). 



.At first the tantalum pens lost more than double the 

 permitted weight, but it was found that slightly oxidising 

 the surface of the pens hardens them so much that they 

 only lose 0-8 mg. by the 10 km. test. By weight this 

 is still more than is permitted for steel pens, but having- 

 regard to the specific weights of the two substances the 

 actual volumetric abrasion of the tantalum pen is the lesser 

 of the two. 



.Although only the surface of the pens had been oxidised, 

 it was found that the rate of abrasion remained the same, 

 for the whole length of 10 km., when it was expected thaf 

 this rate would increase materially after the skin of oxide 

 had been ground off. 



.Advantage was taken of this circumstance when an 

 inauiry was received from India as to whether it would be 

 possible to maimfacture cataract knives for oculists out- 

 of tantalum. The qualities demanded of such a knife are 

 that its blade should be (i) intensely hard, so as to be 

 able to acquire a very sharp edge of great smoothness, and' 

 to retain this fine edge for a long time ; (2) very tough, 

 without any tendency to bend; (3) chemically and 

 mechanically stable, so that it can be easily sterilised and 

 that it is not liable to rust ; (4) capable of acquiring a 

 high polish. Manufacturing such a blade out_ of pure 

 tantalum, and slightly oxidising it before polishing it, 

 appears to fulfil these stringent conditions, but as the 

 knife, which is on the table, has not yet been actually 

 tried for an operation, it can only serve to demonstrate 

 the similarity of tantalum to steel for such purposes. 



Another field for the application of tantalum may be 

 found in the supply of dental instruments, owing to its 

 immunity from chemical changes, but beyond showing two- 

 cases of 'such appliances there is no necessity to go further 

 into details. While possessing all these qualities of a true 

 metal, tantalum has some others which rather limit its 

 usefulness. When heated to a dull red heat it absorbs 

 gases greedily, especially hydrogen and nitrogen, and by 

 combining with them 'it loses its tensile strength and 

 becomes brittle. 



Here are three pieces of tantalum wire taken from the 

 same coil ; one of them has been heated in an atmosphere 

 of nitrogen, the other in hydrogen, and the third has not 

 been interfered w^ith. The consequence is that the latter- 

 has retained its strength, while the former have become 

 brittle and useless. On heating tantalum in air. it shows- 



