64 ON MAGNITUDE [ch. 



A bacillus of ordinary size is, say, 1 /x in length. The length (or 

 height) of a man is about a million and three-quarter times as great, 

 i.e. 1-75 metres, or 1-75 x 10^ /x; and the mass of the man is in the 

 neighbourhood of 5 x 10^® (five million, million, milHon) times 

 greater than that of the bacillus. If we ask whether there may not 

 exist organisms as much less than the bacillus as the bacillus is less 

 than the man, it is easy to reply that this is quite impossible, for we 

 are rapidly approaching a point where the question of molecular 

 dimensions, and of the ultimate divisibility of matter, obtrudes 

 itself as a crucial factor in the case. Clerk Maxwell dealt with this 

 matter seventy years ago, in his celebrated article Atom"^. KoUi 

 (or Colley), a Russian chemist, declared in 1893 that the head of a 

 spermatozoon could hold no more than a few protein molecules ; and 

 Errera, ten years later, discussed the same topic with great ingenuity f. 

 But it needs no elaborate calculation to convince us that the smaller 

 bacteria or micrococci nearly approach the smallest magnitudes 

 which we can conceive to have an organised structure. A few small 

 bacteria are the smallest of visible organisms, and a minute species 

 associated with influenza, B. pneumosinter, is said to be the least 

 of them all. Its size is of the order of 0-1 /x, or rather less; and 

 here we are in close touch with the utmost limits of microscopic 

 vision, for the wave-lengths of visible light run only from about 

 400 to 700 m/x. The largest of the bacteria, B. megatherium, larger 

 than the well-known B. anthracis of splenic fever, has much the 

 same proportion to the least as an elephant to a guinea-pig {. 



Size of body is no mere accident. Man, respiring as he does, 

 cannot be as small as an insect, nor vice versa; only now and then, 

 as in the Goliath beetle, do the sizes of mouse and beetle meet and 

 overlap. The descending scale of mammals stops short at a weight 

 of about 5 grams, that of beetles at a length of about half a milli- 

 metre, and every group of animals has its upper and its lower 

 limitations of size. So, not far from the lower limit of our vision, 

 does the long series of bacteria come to an end. There remain still 

 smaller particles which the ultra-microscope in part reveals; and 



* Encyclopaedia Britannica, 9th edition, 1875. 



f Leo Errera, Siir la limite de la petitesse des organismes, BvlL Soc. Roy. des 

 Sc. me'd. et nat. de Bruxelles, 1903; Recueil d'osnvres {Physiologie gen^rale), p. 325. 



I Cf. A. E. Boycott, The transition from live to dead, Proc. R. Soc. of Medicine, 

 XXII {Pathology), pp. 55-69, 1928. 



