May 3, 1900] 



NA TURE 



21 



In order to prepare non-splintering silica from native masses 

 of rock crystal, the latter must be heated in a Bunsen flame, 

 unless they are already perfectly clean, until the outer impure 

 layers can be removed easily by a blow from an iron pestle or 

 hammer. The clean masses of silica must then be heated in a 

 vessel containing boiling water for some time, and dropped 

 whilst hot into clean cold water. This treatment will cause the 

 masses to crack to such an extent that they may easily be 

 broken into fragments of convenient dimensions by sharp blows 

 from a clean hammer. When the material has thus been broken 

 up, the fragments must be examined one by one, and all those 

 which contain foreign matter must be rejected. Finally, the 

 selected fragments must be heated to a yellow-red heat in a 

 platinum dish, and then quickly thrown into deep cylinders con- 

 taining cold distilled water. After the quartz has been treated 

 in this manner twice, it will be found to be semi-opaque 

 and very much like a white enamel in appearance. It 

 may now be brought safely into the oxy-gas flame, or be pressed 

 suddenly against masses of white-hot plastic silica without any 

 preliminary heating, such as is necessary in the case of the 

 natural quartz. These processes do not occupy much time, and 

 the use of the prepared material saves a great deal of time and 

 trouble at the subsequent stages. We have tried unprepared 

 opal and natural cloudy quartz, but both these splinter badly. 



The Blowpipe. — We have worked silica both in the flame of 

 an ordinary " blow through " jet, and in the flame of a good 

 •'mixed gas" burner. We find the latter gives by far the 

 more satisfactory results. The large " blow through " burners, 

 such as may be used for welding and melting iron, or for melting 

 platinum, do not give satisfactory results, from an economical 

 point of view, with silica. 



Some necessary precaiilioiis. — In working silica it is neces- 

 sary to use very dark glasses to protect the eyes. The darkest 

 glasses usually supplied by spectacle makers are not, in our expe- 

 rience, satisfactory. We use spectacles made specially from 

 glass so strongly darkened, that it is difticult at first to work 

 with them at all. We lay some stress on this matter, as we are 

 satisfied that want of care in selecting the spectacles would be 

 likely to result in injury to the sight of any one who should 

 work silica before the blowpipe frequently, and for long spells. 



Relative difficulty of working Glass and Silica. — The 

 fashioning of apparatus from silica before the blowpipe is expen- 

 sive, for the consumption of oxygen is large, and it demands 

 some patience to build up large pieces of apparatus from 

 shapeless masses of quartz. But owing to the remarkable fact 

 that properly prepared silica, and also silica rendered vitreous 

 by fusion, may be plunged directly into the hottest part of the 

 oxy-gas flame, and afterwards be suddenly cooled, and reheated 

 and recooled, apparently as frequently as one pleases, without 

 any risk of its cracking, it is really very much easier to manipu- 

 late silica than any variety of glass. The most careless and most 

 inexperienced worker runs no risk of breaking his apparatus 

 through want of skill in managing the flame, or through the 

 exigencies of his affairs compelling him to put aside half-finished 

 work. It is important, however, to apply the flame to the 

 opaque prepared silica, in the first instance, in such a way as to 

 avoid the forming of air bubbles. Our practice is to heat 

 first the lowest surface of each fresh mass of silica, and to take 

 care that fusion proceeds regularly from below upwards. If this 

 be done, a perfectly clear glass-like product is obtained. 



Silica is very liable to exhibit a phenomenon resembling 

 devitrification, especially at the earlier stages before the traces 

 of sodium and lithium, which seem to be present in most quartz, 

 have been expelled. In order to avoid permanent injury to 

 the finished work from this cause, care must be taken to employ 

 a quiet flame. If this be done, any devitrification that may 

 appear will be removed easily by reheating the disfigured 

 1 surfaces. 



Jo make Silica Tubes.- Before one commences to construct 

 apparatus of silica it is well to prepare a stock of the vitreous 

 material in the form of rods about i mm. in diameter. These 

 j are made by holding a small lump of non-splintering silica in the 

 flame, by means of forceps with platinum tips, so as to melt one 

 corner of the mass, pressing a second fragment of the material 

 against the heated spot till the two adhere, heating the second 

 portion from below upwards until it assumes a clear vitreous 

 appearance, then adding a third fragment of silica to the second, 

 a fourth to the third, and so on, until an irregular rod has 

 been formed. Finally, this irregular rod must be reheated 

 in small sections at a time, and drawn out to the desired extent. 



NO. 1592. VOL. 62I 



These rods are easily made by any one ; a capable laboratory 

 boy will produce about a score of rods 20 cm. long in an hour, 

 after a few days' practice at the work ; but his consumption of 

 oxygen must be watched closely. The platinum tongs do not 

 suffer much if one works in the manner described, for after the 

 first start off they are only used to press cold fragments of silica 

 against the fused ends of the growing rods. Our forceps have 

 been used by four beginners, and are quite unharmed after 

 several years. 



When a supply of the rods of vitreous silica has been pre 

 pared, bind a few of them, at their ends, with fine platinum 

 wire round a rod of platinum I to i'5 mm. in diameter; heat 

 the silica cautiously till the rods adhere to one another, and then 

 withdraw the platinum core. If the tube is not perfect, add bits 

 of silica at the defective places and reheat them. Close one end 

 of the rough tube thus produced and blow a small bulb upon the 

 closed end, proceeding in the manner employed for producing 

 glass bulbs. Heat the bottom of the bulb, attach a rod of 

 silica to it, reheat the whole bulb and then draw it out into a 

 tube. Blow a fresh bulb at one end of the fine tube thus made, 

 and draw this out in its turn until the tube is six or seven cm. 

 in length. By the time this is accomplished the worker will 

 have discovered that the hottest spot in his oxy-gas flame is just 

 inside the tip of the inner cone, but not too near the orifice of 

 the jet ; and after this, if he can perform the simpler operations 

 of glass working, he will, with a few weeks' practice, find it 

 easy to make larger apparatus by following the simple instruc- 

 tions given below. 



The chief difficulty met with when one wishes to make large 

 bulbs, tubes, &c., is due to the fact that the only thoroughly 

 satisfactory burners give comparatively small flames, and that it 

 is only the hottest parts of these flames that give the desired 

 results. There is no doubt, however, that suitable combinations 

 of small burners could be contrived if they should be demanded, 

 for the production of apparatus of really considerable dimensions. 



In order to convert a small bulb of silica into a large tube, 

 proceed as follows : — ^Ileat one end of a fine rod of vitreous 

 silica, and when it is in the plastic state apply it to the bulb at 

 the point c. Then soften the adjacent parts of the rod and 

 allow them to fall upon the bulb so as to form a ring c B, attached 

 to the bulb. Heat the end of the bulb and c B till the silica 

 softens, then blow out the end in the usual manner. If this process 



is repeated the bulb will first become ovate and then form a short 

 tube which can be lengthened, practically speaking, indefinitely. 

 Tubes of I '5 cm. diameter and of considerable length are easily 

 made in this way by a patient person. It does not answer to 

 add lumps of silica at E and then to blow them out ; we had no 

 success in working silica till we abandoned that method. The 

 sides of a tube formed in that way are too thin, and blow-holes 

 constantly form in them. The tubes are easily thickened, when 

 necessary, by adding rings of silica, reheating these, and blow- 

 ing them to spread the material as one would do when working 

 glass. It is best to blow through a chamber containing potash. 

 If this is connected to the end of the silica tube by india-rubber 

 " valve" tube, one is able to move the silica tube with sufficient 

 freedom. If a large tube is being made, it is best to blow out 

 the softened material whilst it is still in the hottest part of the 

 flame, but smaller objects may be transferred to the less hot 

 parts of the flame with advantage at the moment of blowing. 

 When a comparatively large object must be uniformly heated, it 

 is convenient to place a sheet of silica in front of the flame 

 a little beyond the object to be heated, in order that it may 

 throw back the flames upon those parts of the material which 

 are turned away from the chief source of heat. A suitable plate 

 of silica is easily made by sticking together small, rounded 

 masses of vitrified quartz. 



We find that it is not difficult to produce tubes of various 

 thicknesses and various internal diameters by heating and collaps- 

 ing thin tubes made as described above, and that fine capillaries, 

 " thick millimetre tubes,'' and tubes of two or three millimetres 

 bore, of moderate thickness, can be produced in this way. 

 Thermometer stems are best made by adding rings of silica to 

 small bulbs, thickening them in the flame till their cavities are 



