ULTRAMICROSCOPY 69 



where distillation proceeds from the smaller to the larger drops. 

 The formation of larger crystals in a similar manner may also 

 be noticed. 



Size of the Particles. — The magnitude of the particles present 

 in these coloured glasses has been determined in the following 

 manner : The total mass of gold present in the specimen is 

 known from the amount of gold solution added to the molten 

 glass. The number of particles present in the field of the 

 microscope is counted ; and since the volume of the field is 

 known (see above), the total number of particles, and hence 

 the mass of each, can be determined. In this process it is 

 assumed that all the particles present in the given volume are 

 visible ; that none are too small to be seen, and that no two are 

 so close together as to be indistinguishable. Knowing the mass 

 of a particle, and assuming that the density has the ordinary 

 value and that the shape is some simple figure — for example, 

 a sphere or a cube — the radius of the particle can be determined. 

 The size of the particles depends on the glass examined, the 

 smallest observed having a radius of 3 — 6 x 10 ~ r cm., or 

 3 — 6 fjbfi. It is important to notice that no information as to 

 the shape of the particles can be obtained from direct observa- 

 tion of the patch of diffracted light, which usually appears 

 circular. The method of examining the shape of the particles 

 will be discussed later. 



(2) Coloured Crystals. — Some interesting results have been 

 obtained by Siedentopf by the examination of coloured crystals. 

 Some crystalline substances — e.g. fluor spar and rock-salt — occur 

 in several differently coloured varieties. Recent work seems to 

 indicate that the coloration is due to the presence of small 

 particles of matter. Thus, in the case of fluorite, Cotton and 

 Mouton show that the colouring-matter is formed of grains 

 easily visible with the ultramicroscope, arranged in striae. 



The case of rock-salt is interesting. The blue or violet 

 colours of the natural specimens can be produced artificially 

 by heating colourless crystals of rock-salt in sodium vapour. 

 On further heating these crystals become colourless again. 

 Rock-salt can be coloured in many other ways ; the change is 

 produced by exposure to Rontgen rays, cathode rays, or to the 

 action of the radiations from radium salts. As a result of a 

 series of experiments on the subject, Siedentopf and Goldstein 

 have come to the conclusion that the cause of the coloration is 



