232 CHEMICAL DISCOVERY AND INVENTION 



illumination. The size of the particles inspected cannot be 

 observed directly, but only inferred from a knowledge of the 

 amount of solid contained in a known bulk of the liquid and 

 the number of particles which can be counted in a measured 

 space. 



Ordinary microscopes may give a magnification amounting, 

 under favourable circumstances, to as much as 3000 diameters, 

 and so render visible objects which have a size represented by 

 about one ten-thousandth of a millimetre across. The ultra- 

 microscope is said to be capable of distinguishing objects having 

 a diameter one-tenth of this with light from an arc lamp, and 

 considerably smaller when illuminated with the brightest summer 

 sun. These minute measures are usually indicated by the sym- 

 bols IUL and mu. which stand respectively for one-thousandth 

 and one-millionth of a millimetre. 



It is estimated that the diameters of gaseous molecules must 

 lie somewhere between 0-1 to 0-5 /x/x. Hence there appears little 

 probability of ever rendering these visible, for the finest particles 

 discernible in suspension have a diameter equal to 0-001 //, or 

 1 /x/>t. But though separate individual molecules may never be 

 revealed to mortal eye, observations on some of these very 

 small particles afford a vision which gives a lively stimulus to 

 the imagination. 



It has long been known that very small particles of any solid 

 when suspended in water are seen under the microscope to be 

 in a state of movement. This has no relation to the composition 

 of the solid, but is dependent solely on the size of the particles 

 which must not exceed 3 ^ in diameter. The movement was 

 observed originally by Dr. Robert Brown, the botanist, in 

 1827-8, and hence is commonly referred to as the Brownian 

 movement. The cause of it was long doubtful, but there seems 

 to be now a reasonable explanation which is generally accepted. 



For the study of these movements it has been found con- 

 venient to prepare certain emulsions, 1 by which are to be under- 

 stood milky liquids, which are formed when alcoholic solutions 

 of resin or similar substances are mixed with a large volume of 

 water. The resin being insoluble, or but slightly soluble in water, 

 is thrown out in the form of minute spherical drops, of which the 

 greater part remain for a long time in suspension. Gamboge is 

 a resin familiar as a w^ater-colour pigment which lends itself to 

 1 Milk is the most familiar example of an emulsion. 



