388 BIOT ON THE EMPLOYMENT OF POLARIZED LIGHT 
particle of acid is propagated indefinitely to all the particles 
of water surrounding it, either directly, or by communication, 
like magnetic action, and like that, with an energy decreasing 
in proportion as the mass of water affected augments, without 
limit. : 
73. Lastly, in the fourth section I have considered the effects 
produced by aqueous solutions of tartaric acid of constant 
strengths, when gradually increasing quantities of boracie acid 
are successively introduced. I have shown that, in this case 
again, the primitive aqueous solution combines entirely with the 
boracic acid introduced, forming with it a uniform ternary system, 
of proportions continuously variable, whose progressive change, 
indicated by the optical effects, follows laws similar to those 
which the progressive combinations of tartaric acid with water 
alone had presented. 
74, These experiments, joined to many others which I have 
inserted in the Mémoires and the Comptes Rendus des Séances 
de l’ Académie des Sciences, or which have been made by differ- 
ent observers, evidently show, I think, how much the observa- 
tion of the molecular rotatory powers may serve to throw light 
upon the most abstract questions of chemical mechanics, which 
have hitherto evaded every experimental process. Thus, in many 
cases, it furnishes distinctive molecular characters, not less 
evident than certain, between substances which appear exactly 
isomeric when merely compared by the analyses of their sensible 
masses. What is there more marked, for example, than the 
distinction of tartaric acid and paratartaric acid; the first acting 
by rotation on polarized light, and carrying this power into all 
the combinations into which it enters; while the second, either 
isolated or combined, is absolutely without action? The essen- 
tial oils and their chemical modifications present a multitude of 
similar cases. 
75. Again, we owe to optical processes our first accurate no- 
tions respecting the nature of the substance constituting the 
kinds of starch which may be extracted from the cellular tissue 
of a great number of vegetables, and respecting the progress of 
the chemical changes which it may be made to undergo artifi- 
cially. When these pulverulent products are observed under 
the microscope, between two of Nicol’s prisms, whose princi- 
pal sections are at right angles, especially if their action on 
polarized light is rendered more manifest by the interposition 
