76 SCIENCE PROGRESS 



a series of irregular movements, they suddenly traverse the 

 bright field almost as if they were alive, and disappear." 



Brownian Movements in Smoke Fumes. — Some interesting ex- 

 periments have been made by the Italian physicists, Puccianti 

 and Vigezzi, on the Brownian movements exhibited by particles 

 forming fumes or smoke. In some of their work, tobacco 

 smoke, dried by passage over calcium chloride, enters a vessel 

 such as has been described for the examination of liquids. 

 Employing the apparatus of Siedentopf and Szigmondy and 

 using only a small magnification, a large number of bright 

 points is seen ; and even after the motion of the gas has ceased, 

 the particles show the ordinary Brownian movements. The 

 movements are very rapid, and if the particles are not too close 

 — if, for example, they are separated by a distance of about 

 20 fjLfi or 2 x io -0 cm. — they show two movements, one oscilla- 

 tory, the other translator} 7 . The amplitude of the former is 

 often comparatively large, being of the order of io /x. 



Similar observations have been made with fumes of am- 

 monium chloride ; in this case the formation of the chloride 

 from its constituents can be observed. 



In addition to these Brownian movements, other move- 

 ments can be impressed on the particles. Thus Puccianti and 

 Vigezzi connected their vessel with a Helmholtz resonator, and 

 were able to observe the effects of sound vibrations on these 

 suspended particles. It was found that they were set into 

 vibration, each bright spot becoming drawn out into a line of 

 light ; and it has been suggested that, in this way, some in- 

 formation might be directly obtained as to the magnitude of the 

 movements in a sound-wave. It will be seen later that these 

 movements are very similar to those of the particles of a 

 colloidal solution, under the influence of an alternating electric 

 field. 1 These experiments are important, because our know- 

 ledge of the kinetic theory of gases is much greater than 

 in the case of liquids and solids. We will now pass on 

 to consider the evidence as to the nature of the Brownian 

 movements. 



Explanation of the Brownian Movements. — At first it was 

 thought that these movements were due to convection currents 



1 While speaking of these particles in gases, it may be mentioned that de 

 Eroglie (1908) has recently used the ultramicroscope in the examination of the 

 large ions, which are present in the atmosphere. 



