390 



also the remarkable correspondence existing between the optical pro- 

 perties resulting from this structure, and the phenomena of the cir- 

 cular polarization of fluids, which have been accounted for by the ex- 

 istence of molecules of a structure nearly similar. 



The author proceeds to examine the action of these crystals in 

 common, or unpolarized light ; and concludes from his inquiries 

 that each of their diameters polarized the light in the plane passing 

 through itself and the direction of the ray ; so that the whole emer- 

 gent light consists of equal portions polarized in every plane, and 

 according to every diameter of the circle. This action is similar to 

 that which would be exerted by an assemblage of an infinite number 

 of pieces of tourmaline cut into the form of infinitely small sectors, 

 in the direction of the axis, and disposed as radii in a circle. The 

 author considers it probable that the tourmaline itself is an aggregate 

 of acicular crystals of this description, disposed in a direction parallel 

 to its axis, and being in optical contact, as well as in perfect mecha- 

 nical cohesion. 



In a postscript to this paper, an account is given of a new species 

 of Dichroism in crystals, to the discovery of which the author was led 

 by applying to them his peculiar method of observation with polarized 

 light. In these experiments the crystals themselves perform the 

 office of the analysing plate, acting on light previously polarized, and 

 transmitted through a plate of mica. Under these circumstances, 

 the crystals of borax, described in the first part of the paper, when 

 examined with a lens of moderate power, appear beautifully coloured 

 with two complementary colours, according to the position of their 

 axes. These experiments tend to confirm the views of Sir David 

 Brewster and others as to the general cause of the dichroism of cry- 

 stals, which is ascribed to a difference of absorptive energy in differ- 

 ent directions with relation to their axes; arising from a difference of 

 elasticity in these respective directions. 



May 19, 1836. 



RODERICK IMPEY MURCHISON, Esq., Vice-President, 

 in the Chair. 



A paper was read, entitled, " On the valuation of the mechanical 

 effect of Gradients on a line of Railroad." By Peter Barlow, Esq., 

 F.R.S. 



The exact amount of the influence of ascents and descents occurring 

 in the line of a railway on the motion of a load drawn by a locomotive 

 engine having been differently estimated by different persons, the au- 

 thor was induced to investigate the subject. A few observations are 

 premised on the erroneous assumptions which, he conceives, have in 

 general vitiated the results hitherto deduced. The first of these is that 

 the expenditure of power requisite for motion is equal to the resistance 

 to traction; whereas it must always greatly exceed it. No account, 

 he remarks, has been taken of the pressure of the atmosphere on the 

 piston, which the force of the steam has to overcome before it can be 

 available as a moving power. Another source of error has been that 



