and some suggested Uses of the Finest Threads. 493 



Prof. Judd has kindly given me a variety of minerals 

 which I have treated in this manner. Some behave like glass 

 and draw readily into threads, some will not draw until below 

 a certain temperature, and others will not draw at all, being 

 either perfectly fluid like water, or when a little cooler perfectly 

 hard. 



Among those that will not draw at all may be mentioned 

 Sapphire, Ruby, Hornblende, Zircon, Rutile, Kyanite, and 

 Fluorspar. 



Emerald and Almandine will draw, but care is required to 

 obtain the proper temperature. In the case of the Garnet 

 Almandine, if the temperature is too high, the liquid cylinder, 

 if formed, breaks up, and a series of spheres fall on the table 

 in front of the bow. At a slightly lower temperature the 

 thread is formed, but it is beaded at nearly regular intervals 

 for part of its length. 



Several minerals, especially complex silicates as Orthoclase, 

 draw very readily, but that which surpasses all that I have 

 tried at present is Quartz, which, though troublesome in many 

 ways at first, produces threads with certainty. It required 

 far more force to draw quartz threads than had been previously 

 experienced. The arrow, instead of continuing its flight, 

 hardly disturbed by the drag of the thread, invariably fell very 

 low, and was not in general able to travel the whole distance. 

 So great is the force required that I split many arrows before 

 I succeeded at all. I have obtained threads of quartz which 

 are so fine that I believe them to be beyond the power of any 

 possible microscope. Mr. Howes has lent me a j^g-in. Zeiss of 

 excellent definition, and though, on looking at suitable objects, 

 definite images appear to be formed on which are marks 

 corresponding according to the eyepiece-micrometer to yooV^o 

 inch, yet these threads are hopelessly beyond the power of the 

 instrument to define at all. On taking one that tapers rapidly 

 from a size which is easily visible, the image may be traced 

 until it occupies a small fraction of one division, of which 13*4 

 correspond to yooo mcn on ^ ne stage ; then the diffraction 

 bands begin to overlap the image until it is impossible to say 

 what is the edge of the image. Having reached this stage, 

 the thread may be traced on and on round the most marvellous 

 convolutions, the diffraction-fringe now alone appearing at all, 

 but getting fainter and apparently narrower until the end is 

 reached. That a real thing is being looked at is evident, for 

 if the visible end is drawn away the convolutions of fringes 

 travel away in the same direction. It is impossible to say 

 what is the diameter of these threads ; they seem to be certainly 

 less than xooWo mcn f° r some distance from the end. 



