16 REPORT—1852. 
not only doubly refracting and doubly absorbing, but doubly metallic. The metallicity, 
so to speak, of the medium of course alters continuously with the point of the wave 
surface to which the pencil considered belongs, and doubtless is not mathematically 
null even for the ordinary ray. 
If the reflexion be really of a metallic nature, it ought to produce a relative change 
in the phases of vibration of light polarized in and perpendicularly to the plane of 
incidence.‘ This conclusion the author has verified by means of the effect produced 
on the rings of calcareous spar. Since the crystals were too small for individual ex- 
amination in this experiment, the observation was made with a mass of scales depo- 
sited on a flat black surface, and arranged at random as regards the azimuth of their 
principal planes. The direction of the change is the same as in the case of a metal, 
and accordingly the reverse of that which is observed in total internal reflexion. 
In the case of the extraordinary pencil the crystals are least opake with respect 
to red light, and accordingly they are less metallic with respect to red light than to 
light of higher refrangibility. This is shown by the green colour of the reflected 
light when the crystals are immersed in fluid, so that the reflexion which they exhibit 
as a transparent medium is in a good measure destroyed. 
The author has examined the crystals for a change of refrangibility, and found 
that they do not exhibit it. Safflower-red, which possesses metallic optical proper- 
ties, does change the refrangibility of a portion of the incident light; but the yel- 
lowish-green light which this substance reflects is really due to its metallicity and 
not to the change of refrangibility, for the light emitted from the latter cause is red, 
besides which it is totally different in other respects from regularly reflected light. 
In conclusion, the author observed that the general fact of the reflexion of coloured 
polarized pencils had been discovered by Sir David Brewster in the case of chrysam- 
mate of potash*, and in a subsequent communication he had noticed, in the case of 
other crystals, the difference of effect depending upon the azimuth of the plane of 
incidencet. Accordingly, the object of the present communication was merely to 
point out the intimate connexion which exists (at least in the case of the salt of 
quinine) between the coloured reflexion, the double absorption, and the metallic 
properties of the medium. 
Note added during printing.—When the above communication was made to the 
Association, the author was not aware of M. Haidinger’s papers on the subject of 
the coloured reflexion exhibited by certain crystals. The general phenomenon of 
the reflexion of oppositely polarized coloured pencils had in fact been discovered in- 
dependently by M. Haidinger and by Sir David Brewster, in the instances, respectively’, 
of the cyanide of platinum and magnesium, and of the chrysammate of potash. A 
brief notice of the optical properties of the former crystal will be found in Poggen- 
dorff’s ‘Annalen,’ Bd. Ixviii. (1846), S. 302, and further communications from M. 
Haidinger on the subject are contained in several of the subsequent volumes of that 
periodical. The relation of the coloured reflexion to the azimuth of the plane of in- 
cidence was noticed by M. Haidinger from the first. 
On the Theymal Effects of Air rushing through small Apertures. 
By J. P. Jours, F.R.S. and Professor W. Toomson, M.A. F.R.S.Et 
On the Sources of Heat generated by the Galvanic Battery. 
By Professor W. Tuomson, W.A., F.R.S.E. 
It has been stated as an objection to the chemical theory of the galvanic battery, 
that the chemical action being the same in all elements consisting of zinc and any 
less oxidizable metal, their electromotive force ought according to that theory to be 
the same ; which is contrary to experience, the electromotive force of a zinc and tin 
element in dilute sulphuric acid, for instance, being found by Poggendorff to be only 
* Report of the Meeting of the British Association at Southampton, 1846, partii. p. 7. 
T Ibid. Edinburgh, 1847, p. 5. 
} This paper has been published in the Philosophical Magazine for December 1852. 

