1820.] Philosophical Transactions for 1819, Parti. 41 



having an orange-yellow hue, while the colour of the ordinary 

 image is yellowish-white. This difference of colour is related to 

 the axis of the crystal, and increases with the inchnation of the 

 refracted ray to the short diagonal of the rhomb. It is a maxi- 

 mum in the equator, while along the axis the two images have 

 exactly the same colour and intensity. The author shows that 

 there is an interchange of rays. The extraordinary force carries 

 off several of the yellow rays from the ordinary image ; while, at 

 the same time, the ordinary force takes to itself some of the white 

 rays from the extraordinary image. 



When the rhomboid is exposed to polarized light iu the posi- 

 tion in which the ordinary image vanishes, the extraordinary 

 image is orange-yellow, and in the position in which the extraor- 

 dinary image vanishes, the ordinary image is a yelloxvislt-white. It 

 follows from this, that a portion of the ordinary pencil was 

 absorbed in the first position, and a portion of the extraordinary 

 pencil in the second position. 



The author examined coloured crystals of zircon, sapphyr, 

 Tuby, emerald, beryl, rock crystal, amethyst, tourmaline, rubel- 

 lite, idocrase, mellite, phosphate of hme, phosphate of lead, and 

 observed similar appearances. Now these are a great proportion 

 of all the coloured crystals with one axis of double refraction at 

 present known to exist. 



The general phenomena of absorption in crystals with two 

 axes are nearly the same as those with one ; but the quantity of 

 light which the ordinary and extraordinary forces interchange is- 

 regulated by new laws depending on the situation of the incident 

 ray with respect to the two axes of double refraction. The 

 author explains these laws, and gives a table of the different 

 colours resulting from these absorptions in a variety of coloured 

 crystals with two axes. 



The author concludes from his observations that the colouring 

 particles of crystals, instead of being indiscriminately dispersed 

 throughout their mass, have an arrangement related to the ordi- 

 nary and extraordinary forces which they exert upon light. In 

 some specimens, the extraordinary medium is tinged with the 

 same colouring particles, and with the same number of them as 

 the ordinary medium ; but in other specimens of the same 

 mineral, the extraordinary medium is either tinged with a different 

 number of particles of the same colour, or with a colouring 

 matter entirely different from that of the ordinary medium. In 

 certain specimens of topaz, the colouring matter of the one 

 medium is more easily discharged than that of the other. Hence 

 the reason why such topazes become pink when exposed to a 

 red heat. 



III. Observations on the Decomposition of Starch at the Tern- 

 peralure of the Atmosphere hi/ the Action of Air and Water. By 

 Theodore de Saussure, Professor of Mineralogy in the Academy 

 of Geneva, Correspondent of the Uoyal Institute of France, Sec. 



