416 ON CONCRETIONS, SPICULES, ETC. [ch. 



the physical forces which, at work in nature, are bringing about 

 the result which we have succeeded in imitating. In the present 

 case, however, our enquiry is restricted and apparently simplified ; 

 we are seeking in the first instance to obtain by purely chemical 

 means a purely chemical result, and there is little room for 

 ambiguity in our interpretation of the experiment. 



When we find ourselves investigating the forms assumed by 

 chemical compounds under the peculiar circumstances of associa- 

 tion with a living body, and when we find these forms to be 

 characteristic or recognisable, and fomehow different from those 

 which, under other circumstances, the same substance is wont 

 to assume, an analogy presents itself to our minds, captivating 

 though perhaps somewhat remote, between this subject of ours 

 and certain synthetic problems of the organic chemist. There is 

 doubtless an essential difference, as well as a difference of scale, 

 between the visible form of a spicule or concretion and the hypo- 

 thetical form of an individual molecule; but molecular form is 

 a very important concept ; and the chemist has not only succeeded, 

 since the days of Wohler, in synthesising many substances which 

 are characteristically associated with living matter, but his task 

 has included the attempt to account for the molecular forms of 

 certain "asymmetric" substances, glucose, mahc acid and many 

 more, as they occur in nature. These are bodies which, when 

 artificially synthesised, have no optical activity, but which, as we 

 actually find them in organisms, turn (when in solution) the plane 

 of polarised light in one direction or the other; thus dextro- 

 glucose and laevomahc acid are common products of plant 

 metabolism ; but dextromalic acid and laevo-glucose do not occur 

 in nature at all. The optical activity of these bodies depends, 

 as Pasteur shewed more than fifty years ago*, upon the form, 

 right-handed or left-handed, of their molecules, which molecular 

 asymmetry further gives rise to a corresponding right or left- 

 handedness (or enantiomorphism) in the crystalline aggregates. 

 It is a distinct problem in organic or* physiological chemistry, 



* Lectures on the Molecular Asymmetry of Natural Organic Compounds, 

 Chemical Soc. of Paris, 1860, and also in Ostwald's Klassiker d. ex. Wiss. No. 28, 

 and in Alembic Club Reprints, No. 14, Edinburgh, 1897; of. Richardson, G. M., 

 Foundations of Stereochemistry, N. Y. 1901. 



