COLLOIDS 231 



fully shown by fluorescein and by eosine, 1 this effect is still 

 perceptible when the dilution is 20 million times greater, pro- 

 vided the liquid is examined by the aid of a converging beam 

 from an electric arc directed into the body of the liquid. It can 

 be shown that the particles thus made recognisable are still not 

 resolved into separate molecules, for the dilution would still 

 require to be increased some seven or eight thousandfold before 

 the dissolved substance is reduced to one molecule per cubic 

 millimetre. 



Another illustration of minute division, even into separate atoms, 

 has been described in connection with radio-active substances, 

 the escaping atoms making themselves perceptible by their 

 electrical effects and by exciting phosphorescence on certain 

 surfaces. Each atom as it strikes the screen of he spinthariscope 

 produces a separate vivid spark. 



The observation of minute particles, approaching molecular 

 magnitudes in some cases, has been accomplished within recent 

 years by the use of the instrument known as the " ultra-micro- 

 scope." It has long been recognised that however intense a 

 beam of light from the sun or electric arc may be, it remains in- 

 visible until it strikes some surface. It is then scattered more 

 or less, and if any of the scattered rays reach the eye they pro- 

 duce the sensation of vision. Hence the course of a beam of 

 sunlight traversing a room containing dust-laden air is visible, 

 but if the same beam be transmitted through a glass tube or 

 other vessel in which the air has been purified from suspended 

 particles the course of such a beam will be imperceptible. In 

 the ultra-microscope 2 advantage is taken of these facts. The 

 medium holding minute particles diffused through it is examined 

 through a high-power microscope, while a strong beam of light, 

 admitted through a very fine slit, is cast through the space in 

 front of the object glass. As the axis of the microscope is verti- 

 cal while the beam is horizontal it is obvious that no portion of 

 it enters the microscope directly. Hence if anything is seen it 

 is the little rays reflected sideways from the surface of the par- 

 ticles under examination. The image observed therefore de- 

 pends on the power of the microscope and the intensity of the 



1 Everyone has seen the peculiar blue light (fluorescence) exhibited by a 

 watery solution of quinine sulphate. 



2 The ultra-microscope was invented by H. Siedentopf and R. Zsigmondy 

 early in the present century. 



