TRANSACTIONS OF THE SECTIONS. 9 
which Comte pronounced to be insurmountable, and which would have excluded 
from the objects of chemical research any thing lying without the limits of our 
earth. Comte warned us that our knowledge of the planetary worlds was neces- 
sarily limited to their geometrical and mechanical properties—to the nature of their 
movements, and the forces by which they are produced,—and that all inquiry into 
the constituent elements of the planets or their atmospheres was for ever, and by the 
necessities of the case, interdicted to us. But the spectroscope has told quite 
another story. 
But there is another point of contact between optics and chemistry,—another 
spot on the border-land between these two sciences which, I think, promises also 
to be fertile in discovery,—I mean the use of polarized light as an instrument of 
chemical analysis. It is true that the application of this instrument is limited in 
its extent. The physical property on wivel this application depends (namely, the 
power possessed i certain liquids to change the plane of polarization of a trans- 
mitted ray, or, as it is commonly called, the rotatory power) is almost wholly con- 
fined to’ the organic world, and is not universal even there. Still, within this 
limited range, the application of polarized light is capable of solving, or aiding to 
solye, chemical problems which chemistry;proper would probably find very difficult. 
Let me give you two examples. 
1. Is it true that an acid salt is decomposed by solution? Or, taking the ques- 
tion in another form: If to a solution of a neutral salt there be added, atom for 
atom, a quantity of its own acid, does that additional atom of acid enter into com- 
bination, or does it remain free? It has been usually inferred from the thermic 
researches of Dr, Andrews, followed up by Favre, Silbermann, Berthelot, and others, 
that the second alternative is the true one, the solvent being water. Now, if the 
roblem be varied a little by making the solvent spirit, the application of polarized 
ght gives us this important information :— 
If to an alcoholic solution of the ordinary nitrate of quinia there be added an 
additional equivalent of acid, this additional equivalent does enter into combination 
with the nitrate. 
This information leaves to us the alternative of supposing that the ordinary 
nitrate, sulphate, &c. of quinia are not neutral but basic salts, or of admitting that 
an acid salt is not always decomposed by solution, at least in spirit. 
_ 2. When an acid is added to a solution containing two bases, the salts formed 
being also soluble, does the acid divide itself between the bases? and if so, what is 
the law which governs the division ? 
The application of polarized light enables us to solve this question for some of 
the organic bases, proving that there is a continuous partition of the acid, and 
enabling us in one case, and probably in many others, to assign the law according 
to which the partition is aad 
One more instance may suffice to exemplify the advantage which chemistry proper 
has already derived from its union with optics. Itake this instance from the general 
problem of saccharometry. 
We have long known how te analyze, both optically and chemically, a solution 
containing two kinds of sugar, one of which is sucrose. But as each of these methods 
gives but two equations, it is plain that neither is sufficient where the unknown 
quantities are more than two. If, then, as is very commonly the case, there are 
present in the solution three kinds of sugar, we cannot obtain a complete analysis, 
either from optics or from chemistry. But, as Dr. Apjohn has recently shown, this 
problem, insoluble by either method taken alone, is readily solved by a combination 
of both methods. An important step is thus made in the application of optics to 
chemistry. Instead of merely giving to chemistry a new solution of a problem 
which chemistry could solve without any assistance, optics has aided chemistry to. 
solve a problem which chemistry unaided might have found very difficult. 
But it is time that I should bring these remarks to a close ; and I recur, in con- 
clusion, to a thought which my subject has already suggested. 
Let none presume to fix the bounds of Science. “Hitherto shalt thou come, but 
no further ”—that sentence is not for man, Not by our own powers, not by the 
powers of our generation, not even by our conceptions of possibility, may we limit 
the march of scientific discovery, ‘'o us, labourers in that great field, it is giver 
1874. g 
