ULTRAMICROSCOPIC BODIES 51 



background, smaller objects than these can be detected, 

 though they cannot be clearly defined. They are seen 

 as tiny blurs of light owing to diffraction of the rays 

 round the ultramicroscopic particles. By this method 

 particles of about '005 /z (= 5 JLC/X) * may be detected, 

 which we know from calculations based on other data 

 is about the size of the very large molecules of soluble 

 starch. 2 The disperse particles of very fine sols run 

 down to perhaps 2*5 /z/z, the size of the molecules of 

 haemoglobin, the red pigment of the blood. The 

 molecules and ions (single atoms, or small groups of 

 atoms, into which a fine solute may be dissociated) 

 of true (crystalloid) solutions are much smaller than 

 this, from '5 /zju- (alcohol molecule) down to perhaps 

 *i /z/i, which is about the diameter of a hydrogen ion 

 (separate atom of hydrogen). 



To give some more easily comprehensible notion of 

 the relative sizes of these exceedingly minute objects : 

 if we imagine the hydrogen ion magnified a million 

 times, so that it was just visible to the naked eye as 

 the smallest possible ink dot on a sheet of paper, then 

 a very large organic molecule (such as might just be 

 detected with the ultramicroscope), if it were magnified 

 to the same extent, would be about the size of a small 

 pea, and a very small bacterium (the smallest organisms 

 that are just visible with the microscope) would be 

 twice the diameter of a football. Represented on this 

 scale a real pea would appear more than three miles 

 in diameter. 



Internal Surface of Colloids. The very large number 

 of particles or droplets forming the disperse phase of 



1 ftp is one-thousandth of i p, and is the unit of ultramicroscopic 

 measurement. 



1 Soluble starch is a simpler form of starch produced on boiling 

 with dilute hydrochloric acid and forming a colloid sol with water. 



