fO- Katural-Pkilosophical Collections. 



On the Prodttction of Double Refraction by simple pressure ; and on the 

 origin of the doubly refracting structure ; by David Brewster, L.L.D. 

 F.R.S.L. ^ £. 



Dr. Brewster has lately read a paper before the Roy. Soc. of London, entitled 

 " On the production of regular double refraction, in the molecules of bodies, by 

 simple pressure ; with observations on the origin of the doubly refracting struC' 

 ture. — The author has already shown, in former papers which have appeared in the 

 Philosophical Transactions, that the phenomena of double refraction may be pro- 

 duced artificially by effecting certain changes in the mechanical condition of hard 

 Sind of soft bodies. In all these cases, he observes, the phenomena are entirely 

 different from those of regular crystals ; and in none of them is the doubly refract- 

 ing force a function of the angle which the incident ray forms with one or more 

 axes given in position. In the year 1815 he noticed the depolarizing properties 

 of a thin film, or of a mixture of resin and white wax, compressed between two 

 pieces of glass. Accidentally meeting with the specimen which had originally 

 been the subject of this observation, he found that after fifteen years it still re- 

 tained this property of depolarization, and was induced to pursue the inquiry to 

 which it led. He varied the proportions of the ingredients, and observed in the 

 different cases the modifications produced in the phenomena by employing vari» 

 ous degrees of pressure. He found that in every point their existed an axis of 

 double refraction perpendicular to the plane of the film ; and that the doubly re- 

 fracting force varied with the inclination of the incident ray to this axis : just as 

 happens with all regular uni-axal crystals. He infers from his observations, that 

 the property of uni-axal double refraction is communicated to the molecules simply 

 by the agency of pressure ; for in all cases where pressure has not operated, the 

 aggregate does not exhibit this property. These effects are precisely the same as 

 those which would be produced by subjecting elastic spheres to a regular com- 

 pressing force ; the axis of pressure becoming an axis of positive double refrac- 

 tion ; while extension, on the contrary, produces a negative axis. 



From the consideration of the preceding facts, the author is led to a very simple 

 explanation of the origin and general phenomena of double refraction in regular 

 crystals. He considers this property as not being inherent in the molecules them- 

 selves ; but as resulting from their compression either by an extraneous force or by 

 their power of inherent attraction of aggregation. The phenomena of crystalliza- 

 tion and of cleavage prove that the molecules of crystals have several axes of at- 

 traction or lines, along which they are most powerfully attracted, and in the di- 

 rections of which they cohere with different degrees of force. Guided by the in- 

 dications of hemitrope forms, and supposing the molecules to be spherical or 

 spheroidal ; it is inferred that these axes are three in number, and at right angles 

 to each other, and that they are related in position to the geometrical axis of the 

 primitive form. In like manner, the phenomena of double refraction are related 

 to the same axis of the primitive form ; and may be all rigorously calculated by 

 a reference to three rectangular axes. The author pursues the consequences of 

 these principles in their application to various kinds of crystals. It follows, from 

 this theory, that the forms of the ultimate molecules of crystals existing separate- 

 ly, determines within certain limits the primitive form to which they belong, 

 while the doubly refracting structure and the precise form of the crystal are si- 

 multaneously produced by the action of the forces of aggregation. These 

 views receive a remarkable confinnation in the doubly refracting structure which 

 the author discovered in chabasite ; and they also enable us to understand the 

 nature of that influence which heat produces on doubly refracting crystals, as dis- 

 covered by Professor Mitscherlich. The optical phenomena exhibited by fluids 

 under the influence of heat and pressure, and by crystals exposed to compressing 

 or dilating forces, are also in perfect conformity with the above views, and would 

 in themselves have been sufficient to establish the principle that the forces of 

 double refraction are not resident in the molecules themselves, but are the imme- 

 diate result of those mechanical forces by which these molecules constitute solid 

 bodies. — Lit. Gazette. 



