42 ON MAGNITUDE [ch. 



According to the most recent estimates, the weight of the 

 hydrogen molecule is somewhat less than that on which Errera 

 based his calculations, namely about 16 x 10"^^ mgms. and 

 according to this value, our micrococcus would contain just about 

 27,000 albumin molecules. In other words, whichever determina- 

 tion we accept, we see that an organism one-tenth as large as our 

 micrococcus, in hnear dimensions, would only contain some thirty 

 molecules of albumin ; or, in other words, our micrococcus is only 

 about thirty times as large, in hnear dimensions, as a single albumin 

 molecule *. 



We must doubtless make large allowances for uncertainty in 

 the assumptions and estimates upon which these calculations are 

 based ; and we must also remember that the data with which the 

 physicist provides us in regard to molecular magnitudes are, to 

 a very great extent, tnaximal values, above which the molecular 

 magnitude (or rather the sphere of the molecule's range of motion) 

 is not Ukely to he : but below which there is a greater element of 

 uncertainty as to its possibly greater minuteness. But nevertheless^ 

 when we shall have made all reasonable allowances for uncertainty 

 upon the physical side, it will still be clear that the smallest known 

 bodies which are described as organisms draw nigh towards 

 molecular magnitudes, and we must recognise that the subdivision 

 of the organism cannot proceed to an indefinite extent, and in all 

 probability cannot go very much further than it appears to have 

 done in these already discovered forms. For, even after giving 

 all due regard to the complexity of our unit (that is to say the 

 albumin-molecule), with all the increased possibilities of inter- 

 relation with its neighbours which this complexity impHes, we 

 cannot but see that physiologically, and comparatively speaking, 

 we have come down to a very simple thing. 



While such considerations as these, based on the chemical 

 composition of the organism, teach us that there must be a definite 

 lower hmit to its magnitude, other considerations of a purely 

 physical kind lead us to the same conclusion. For our discussion 

 of the principle of simihtude has already taught us that, long 

 before we reach these almost infinitesimal magnitudes, the 



* McKendrick arrived at a still lower estimate, of about 1250 proteid molecules 

 in the minutest organisms. Brif. Ass. Rep. 1901, p. 808. 



