II] OF MOLECULAR MAGNITUDES 65 



here or hereabouts are said to come the so-called viruses or "filter- 

 passers," brought within our ken by the maladies, such as hydro- 

 phobia, or foot-and-mouth disease, or the mosaic diseases of tobacco 

 and potato, to which they give rise. These minute particles, of the 

 order of one-tenth the diameter of our smallest bacteria, have no 

 diffusible contents, no included water — whereby they differ from 

 every living thing. They appear to be inert colloidal (or even 

 crystalloid) aggregates of a nucleo-protein, of perhaps ten times the 

 diameter of an ordinary protein-molecule, and not much larger than 

 the giant molecules of haemoglobin or haemocyanin *. 



Bejerinck called such a virus a contagium vivum; "infective 

 nucleo-protein" is a newer name. We have stepped down, by a 

 single step, from Hving to non-Hving things, from bacterial dimen- 

 sions to the molecular magnitudes of protein chemistry. And we 

 begin to suspect that the virus-diseases are not due to an "organism, 

 capable of physiological reproduction and multiphcation, but to a 

 mere specific chemical substance, capable of catalysing pre-existing 

 materials and thereby producing more and more molecules hke 

 itself. The spread of the virus in a plant would then be a mere 

 autocatalysis, not involving the transport of matter, but only a 

 progressive change of state in substances already there f." 



But, after all, a simple tabulation is all we need to shew how 

 nearly the least of organisms approach to molecular magnitudes. 

 The same table will suffice to shew how each main group of animals 

 has its mean and characteristic size, and a range on either side, 

 sometimes greater and sometimes less. 



Our table of magnitudes is no mere catalogue of isolated facts, 

 but goes deep into the relation between the creature and its world. 

 A certain range, and a- narrow one, contains mouse and elephant, 

 and all whose business it is to walk and run ; this is our own world, 



* Cf. Svedberg, Journ. Am. Chem. Soc. XLviir, p. 30, 1926. According to the 

 Foot-and-Mouth Disease Research Committee {oth Report, 1937), the foot-and- 

 mouth virus has a diameter, determined by graded filters, of 8-12m/i; while 

 Kenneth Smith and W. D. MacClement {Proc. R.S. (B), cxxv, p. 296, 1938) calculate 

 for certain others a diameter of no more than 4m^, or less than a molecule of 

 haemocyanin. 



t H. H. Dixon, Croonian lecture on the transport of substances in plants, 

 Proc. R.S. (B), vol. cxxv, pp. 22, 23, 1938. 



