42 COLLOIDS IN BIOLOGY AND MEDICINE 



held back are larger than the meshes of the sieve. If we know the 

 size of the meshes and have several sieves with meshes of different 

 size, it is easy to determine the average size of the grains by letting 

 them pass through the different sieves. H. BECHHOLD'S determina- 

 tion of the size of the particles depends on this principle. Ultra/liters 

 (jelly filters) with different sized pores serve as sieves (see pp. 99 et seq.). 

 Since there are several methods for measuring the size of the pores 

 (see p. 100), it is possible to determine definite limits for the size of 

 the colloid particles. 1 



Are the particles thus found identical with molecules? 



In the case of metal sols we can immediately say, no. We know 

 the molecular weight of metals and understand from it that there is 

 no prospect of directly seeing the molecules of the elements with 

 our present instruments. According to E. RIECKE gold particles 

 of 1 MM diameter have a molecular weight of 300,000, but the molec- 

 ular weight of gold is probably only 197, and the smallest particles 

 we can see have a diameter of 5 MM- It follows, therefore, that 

 every recognizable ultramicroscopic particle consists of thousands of 

 molecules. 



What are the facts in the case of particles whose size is determinate 

 by ultrafiltration? Since albumin, starch, etc., have unusually large 

 molecules, it is probable that in them the molecule and particle size, 

 as determined by ultrafiltration, are identical. This is all the more 

 likely since these bio-colloids, like crystalloids, are distributed by 

 means of the action of the solvent, whereas the metal hydrosols are 

 brought into such minute divisions only by artificial means. 



But what is a molecule? It is the smallest portion of a compound 

 or of an element that may exist alone. If we split a molecule of 

 common salt we no longer have a molecule of NaCl but an atom 

 of Na and an atom of Cl. If we divide an albumin molecule, we 

 still have complicated atom complexes but we have albumin no 

 longer. Molecular weight is the weight of a molecule compared to 

 that of an atom of hydrogen which equals unity. Consequently we 

 are measuring not absolute, but relative sizes. The molecular 

 weight is determined by purely chemical means. If, for example, 

 we find in sodium benzoate, that there are 7 atoms of carbon 

 (7 X 12 = 84), 5 atoms of hydrogen (5X1 = 5), 2 atoms of oxygen 

 (2 X 16 = 32) and one atom of sodium (1 X 23 = 23), we should 

 know that the molecular weight must be at least 144, because half 

 atoms do not exist. The molecular weight might in fact be two 

 or three times as large, which would have to be determined by other 



1 [J. Alexander has recently proposed measurement of particle size by high 

 speed centrifugation, "ultracentrifugation." Tr.] 



