119 



•dividing engine at the rate of many hundreds to the inch. Pieces 

 of glass so ruled are known as diffraction gratings and used in the 

 production of pure spectra. Gratings have been prepared having 

 4,500 parallel and equidistant lines drawn within a square of fin. 

 side. Yet another use is in wire-drawing. A hole is drilled 

 through a diamond and the wire drawn through it. 



I now turn to another division of my subject. Carbon mani- 

 fests itself to us not only in the imperial diamond of magnificent 

 lustre and " indomitable, unspeakable hardness," but as the soft 

 and black graphite. German philosophy could hardly supply us 

 with such another example of the synthesis of apparent irrecon- 

 cileables into a unity. Carbon is hard and white, but it is also 

 soft and black. With the geology and geographical distribution 

 of graphite other lecturers have dealt, but I may remind you that 

 it is obtained from Siberia, Ceylon, Bohemia, Bavaria, North 

 Italy, and the U.S.A. The excellent graphite of Borrowdale in 

 Cumberland has, unfortunately, been exhausted. It is also made 

 artificially and, since the introduction of the electric furnace into 

 technical use, this artificial graphite is of considerable import- 

 ance. The name graphite indicates its use for writing, other 

 names which still cling to it, black lead and plumbago, are relics 

 of a time when its chemical nature was not understood. Lead 

 pencils, as we still call them, appear to have been made originally 

 from slips of natural graphite cut out from the native mineral and 

 enclosed in a wooden cylinder. Natural graphite good enough to 

 be used in this way was, I believe, once found in Borrowdale, but 

 the supply was very small and soon exhausted. It was found, 

 however, that satisfactory pencils could be made from graphite of 

 less super-excellent quality. Briefly, the process is as follows : — 

 The graphite is ground, washed and sifted so as to remove all 

 traces of grit. The purified product is mixed with clay that has 

 been similarly treated and the mixture, with the aid of some other 

 ingredient to bind it together, is made into a paste. This is 

 squeezed through a hole and issues as a soft thread of the proper 

 size. It is next dried and afterwards baked, thus becoming hard 

 and suitable for insertion into a groove, made by a machine, in a 

 piece of wood having the shape of a half cylinder. A similar 

 piece of wood is glued to this and the cylinders are cut up into the 

 conventional lengths, stained, polished and stamped with the 

 maker's name, etc. According to the proportion of graphite and 

 clay the pencils are of various grades, known in England by the 

 letters B and H or these in combination. Thus B = black, H = 

 hard, HB = hard and black, BB = very black, HH = very hard 

 and so on. Besides their legitimate use in writing lead pencils 

 come in for other purposes. The "lead " of a hard pencil may 

 be used in the chemical laboratory as a cheap substitute for 

 platinum wire in performing what are called flame reactions. 



The use in writing depends, of course, on the comparative 

 softness and blackness of graphite, properties which fit it for 

 making marks on paper, and the ease with which such marks can, 



