THE COLLOIDAL STATE 79 



later, to have a radius of about one-millionth of a centimetre. A sphere of gold 

 of one- tenth of a centimetre radius has a surface of 0-126 sq. cm., while the 

 surface of the same mass, if subdivided to the above colloidal dimensions, would 

 have a surface of about 100 sq. m., or be multiplied by ten millions. 



It will occur to the reader that we are very near molecular dimensions in the case of these 

 finest particles. In fact, Siedentopf and Zsigmondy obtained gold hydrosols with particles of 

 less than 6 ^ in diameter (fj. is O'OOl mm., and /JL/J. is one-thousandth of this, i.e., one- 

 millionth of a millimetre), while starch is stated by Lobry de Bruyn and Wolff (1904) to have 

 a molecular diameter of 5 iJ.fi, and even carbon dioxide has a value of 0"29 fn/j. (Nernst, 1911, 

 p. 434). 



In practice it is found that Graham's criterion of not passing through parch- 

 ment paper is the most satisfactory one for deciding whether a particular solution 

 is a colloidal one. This property goes together with the various other properties 

 dependent on surface development, although it must be admitted that it is some- 

 what arbitrary to fix the point at a definite dimension. Indeed, there are sub- 

 stances on the border line, like certain dyes, which will pass through some samples 

 of parchment paper, but not through others, and these substances are found to 

 possess some of the colloidal characteristics but not all. 



When we are dealing with such things as gold, silica or arsenious sulphide, 

 we know that the size of their particles can only be attained by the aggregation 

 of a number of molecules ; but, as we have just seen, the single molecules of some 

 organic compounds, such as starch, may be of sufficient size to present properties 

 of surface. Haemoglobin does not pass through parchment paper, but measure- 

 ments of its osmotic pressure by Hiiffner and Gansser (1907) have shown that 

 it is present in solution in single molecules. How this is known will be under- 

 stood after Chapter VI. on osmotic pressure has been read. In the case of salts, 

 such as Congo-red or caseinogen in alkaline solution, which are electrolytically 

 dissociated in solution, but of which neither ion passes through parchment paper, 

 complications are present which will be discussed in the next chapter. It may 

 be that the organic ion itself is sufficiently large to possess the properties of 

 the colloidal state, or there may be aggregates of these ions formed. 



THE ULTRA-MICROSCOPE 



Much of the recent progress in knowledge of the colloidal state is due to 

 the use of the ultra-microscope. This method was first described by Siedentopf 

 and Zsigmondy in 1903. Details of the construction of the instrument would 

 be out of place here. The reader is referred to the original paper (1903) or to the 

 book of Zsigmondy (1905, pp. 83-97). Space for the principles only, on which it 

 depends, can be found here. 



It is a matter of common observation that dust particles, completely invisible 

 under ordinary light, become clearly visible in a beam of sunlight. Rayleigh 

 (1899) has shown that to make visible a particle, which is too small to be seen 

 by the highest power of the microscope, merely requires sufficiently intense 

 illumination. It must be remembered that these particles are smaller than 

 the wave lengths of the visible part of the spectrum. For example, the wave 

 length of the D line of sodium is 589 p.^ and the limits of the visible spectrum 

 lie roughly between 700 and 400 pp. Dimensions of such values are high 

 for the particles in a colloidal solution, which may be as small as 6 /xju., as we 

 have seen, although this is an unusually small size. Any object smaller than 

 half the wave length of the light by which it is illuminated cannot be seen 

 in its true forjn and size owing to diffraction. Hereby is set a limit to microscopic 

 observation. A brilliantly-illuminated dust particle in a beam of sunlight 

 is seen as a disc, due to diffracted rays sent off from its surface, and looks 

 much larger than it actually is. 



The Faraday phenomenon in a colloidal solution is similar to that of the 

 motes in a sunbeam. It occurred to Siedentopf and Zsigmondy that if the 

 solution was much diluted and the beam examined by the microscope, placed 

 perpendicularly to its track, so as not to receive the direct light, the diffraction 

 images of the separate particles would be visible. In that form of the ultra- 



