100 PROTOPLASM 



butterfat in milk is 8 /^ across, and a bacterium is between 

 1 and 5 ^ long. Ultramicroscopic particles require a smaller 

 scale, such as was developed for measuring the wave length of 

 light. Knowing that the micron is a thousandth part of a 

 millimeter, and forgetting that its name and symbol were chosen 

 to designate a millionth part of a meter, the symbol ixix was 

 introduced to designate the thousandth part of a ix. The 

 physicist, however, realizing the error in designating a thousandth 

 part by n, uses the symbol m/i, the so-called millimicron, for the 

 thousandth part of a millionth part of a meter, as m stands for a 

 thousandth part. The physicist also uses the symbol /x^t but 

 correctly so, viz. to indicate the millionth part of a millionth part 

 of a meter, the so-called micromicron. 



It is impossible to grasp the true magnitude of such minute 

 dimensions, but some idea of them can be gained if we approach 

 them from things of appreciable size. The following table 

 may help to do this: 



1 meter (m.) = 1,000 millimeters Sound waves are 16 m. to 



17 mm. in length 



1 millimeter (mm.) = 1,000 microns Cells range from 0.15 



mm. to Ifi 



1 micron (/x) = 1,000 millimicrons Colloidal particles range 



between 0.1 ^t and 1 m/t 



1 millimicron (m/^) = 1,000 micromicrons (mm) • Molecules range from 



2.5 mM (protein) to 46 nfi 

 (water) and less 



To this scale, each member of which is a thousand times the 

 one below it, may be added the angstrom unit, A. U., 

 used chiefly in indicating the wave length of light. It is 

 0.1 m// and therefore 100 mm. The light waves of the visible 

 spectrum are 7,500 A. U. long at the red end and 3,900 A. U. 

 long at the violet end. 



Particle size is of importance in the colloidal world in that 

 it determines many essentially colloidal characters, such as 

 surface and stability. To be truly colloidal, a dispersion must 

 be permanent, yet permanency is relative. Gold particles 

 which remain in suspension for five years are certainly colloidal, 

 and particles that settle in five minutes are not. Obviously, 

 no sharp line can be drawn between these limits. Clay particles 

 that settle in five hours or five days are colloidal only in so far 



