32 



REPORT — 1842. 



On Apparatus for applying Circular Polarization to Chemical Inquiries. 

 By Prof. Powell, F.R.S., Sfc. 



The application of the phenomena of circular polarization as characterizing certain 

 liquid solutions, has been fully pointed out by M. Biot, and an apparatus has been 

 devised by him of the most perfect and accurate construction, for ascertaining and 

 measuring the effects in question. That apparatus is however complicated, expensive, 

 and very difficult to adjust, especially for those not familiar with optical experiments : 

 and where the object may be rather general indications than minute measures, it ap- 

 peared to the author susceptible of considerable simplification, which though unim- 

 portant to the more refined optical inquirer, might be valuable to the chemical student. 

 In general the essential parts of any such apparatus are, — 1st, a polarizing plate ; 

 2ud, a tube containing the liquid under examination, so constructed that the polar- 

 ized ray can pass along its axis ; 3rd, an analyser of double-refracting crystal. 



In M. Biot's apparatus the supports, &c. are complex and difficult to adjust. The 

 tubes are of costly construction, each end being covered with plates of glass, so that 

 when filled with liquid, and the glasses screwed on tight, the original luminous 

 image is seen distinctly through a great thickness of the 

 liquid, bounded by truly plane parallel surfaces. To 

 fulfil these conditions is a very troublesome process. The 

 analyser also must consist of calc spar of the greatest 

 purity, cut and adjusted with perfect accuracy to give 

 two truly achromatized images considerably separated. 



The simplified form proposed by the author is as fol- 

 lows : — 



1st. Instead of the tubes with parallel glass ends, &c, 

 he U6es merely common test tubes, having a hemispherical 

 bottom and open at the top. The use of these is of 

 course limited to the vertical position ; and the polarizing 

 plate (P) of black glass must be fixed below at 35V to the 

 axis of the vertical tube (T). 



2ndly. It is consequently necessary to introduce a small 

 silvered mirror (S) to throw the light upon (P) in the 

 proper direction. 



3rdly. The tube must be enclosed in an opake case, 

 and the light admitted at the bottom through a small 

 hole. When the eye looks down it, the image of the lu- 

 minous hole will be very irregular ; but, 



4thly. This evil is remedied by the remaining pecu- 

 liarity in the analyser (A) ; which instead of the double 

 refracting prism, consists simply of a double refracting 

 crystal in its natural state, about three-fourths of an inch 

 in thickness (R), on which the light falls through a hole 

 (h), about one-twentiethof an inch in diameter ; the crystal 

 is enclosed in a tube, in which slides another carrying a 

 small lens (L), which at once magnifies the separation, and 

 forms two distinct circular discs, however irregular the 

 light, and in which all the optical changes are distinctly 

 seen, either by the light of the clouds, or of a lamp, on 

 making the analyser revolve about the axis, and measu- 

 ring its rotation on a divided circular rim round it. 



The supports should be so contrived that the analyser 

 may be adjusted to different heights, to allow of the in- 

 sertion of tubes of different lengths from about six to 

 eighteen or twenty-four inches. 



On some peculiar instances of (so-called) Catalytic Action. By 

 Mr. Mercer. 

 Mr. Mercer had long considered that instances of catalysis were merely examples 

 of chemical affinity exercised under peculiar circumstances. A body never entirely 



