276' CHEMICAL DISCOVERY AND INVENTION 



lime are afterwards removed by steeping the knitted fabric in a 

 weak solution 1 to 2 per cent of hydrochloric acid or acetic 

 acid. The acid is then removed by agitation of the fabric in 

 distilled water, and, if the hose is required to be kept any length 

 of time, this is followed by a bath of dilute ammonia. The 

 stocking is dried at a low temperature by hanging over poles or 

 drawing continuously through a hot chamber through which a 

 current of hot air is blown. It is then cut up, usually on a 

 cutting machine similar to the one illustrated (Fig. 70), which 

 cuts the fabric into pieces of equal length and width, stacks 

 them, and automatically counts them as cut. 



We must now treat the upright and inverted mantles separ- 

 ately. 



The heads of the upright mantles require to be reinforced by 

 stitching on to them a piece of cotton or ramie tulle, with the 

 object of strengthening the head and keeping it to a uniform 

 size. They are then dipped in the previously prepared strong 

 solution of thorium nitrate with about 1 per cent of cerium 

 nitrate and -5 to 1 per cent of other hardening materials, such as 

 aluminium, zirconium, or calcium nitrate, put upon the elevator 

 illustrated in figure 71, and carried through gutta-percha 

 rollers, carefully adjusted to leave exactly the right quantity of 

 thorium nitrate in the mantle. The stocking is then dried upon 

 glass forms shown in figure 72, and when dry the head is rein- 

 forced with a further supply of solution containing a higher 

 proportion of the hardening materials. Such " fixing " fluid 

 consists often of a solution of aluminium, magnesium, and 

 calcium nitrates in water. The mantles are again dried and then 

 sewn with asbestos thread to form the head. 



The next operation is that of burning off the organic matter of 

 the mantles, and at the same time decomposing the nitrates with 

 which they are impregnated. The stocking is first shaped over a 

 wooden form and hung up by means of an iron hook (Fig 73), fired, 

 and allowed to burn until nothing but the white ash of thorium 

 and cerium oxides remains. This ash is now in a loose condition 

 and is extremely fragile. With the help of a burner supplied 

 with either gas or air at a pressure of from 5 to 15 Ibs. per square 

 inch, the mass of oxides is blown out to its correct shape, and, 

 by an up and down movement of the burner inside the mantle, 

 1 the latter is brought to its final state of hardness, the change 

 being due to the partial " fritting " of the more fusible oxides 



