* », Propojiikns re/^eBlng Lighl. — Bttuminsi jOt 



properties of light, and the nature of the forces which bodies exert on it internally, I con- 

 clude at prefent with a fliort fummary of propofitions. But, firft, may I be permitted to ex- 

 prefs a hope, that what has been already attempted (and for which no praife can be claimed 

 farther than what is due to attentive obfervation, according to the rules of the immortal 

 Bacon) may prove acceptable to fuch as love to admire the beautiful regularity of nature, 

 or more particularly to trace her operations, as exhibited in one of the mofl pleafing, moft 

 important, and moft unerring walks of phyfical fcience. 



Propofttion I. The fun's light confifts of parts which differ in degree of refrangity, re- 

 flexity, inflexity, and deflexity ; and the rays which are moft flexible have alfo the greateft 

 refrangity, reflexity, and flexity — or are moft refrangile, reflexile, and flexile. 



Propofttion II. Rays of compound light pafling through the fpheres of flexion, and falling 

 on the bending body, are not feparatcd by their flexibility, either in their approach to, or re- 

 turn from, the body. 



Propofttion III. The colours of thin and thofe of thick plates are precifely of the fame 

 nature, differing only in the thick;nefs of the plate which forms them. 



Propefition IV. The colours of plates are caufed by flexion, and may be produced with- 

 out any tranfmiflion whatever. 



Propofttion V. All the Confequences deducible from the theory a priori are found to fol- 

 low in faft. 



Propofttion VI. The common fringes by flexion (called hitherto the " three fringes") 

 are found to be as numerous as the others. 



Propofttion VII. The unufual image by Iceland cryftal is caufed by fome power inherent 

 in its particles, difl^erent from refradtion, reflexion, and flexion. 



, Propofttion VIII. ' This power refembles refradtion in its degree of action on different 

 rays; but it refembles flexion within the body, in not taking place at a diftance from it, in 

 a£ting as well on perpendicular as on oblique rays, and in its fphere or Ipace of exertion 

 moving with the particles which it attends. 



11. 



Ohfervations on Bituminous Suhftances, with a Defcriptlon of the Varieties of the Elaflic 

 Bitumen. By CHARLES HATCHETTy Efq. F.R.S. Land, and Edin. F.L.S. i^c* 



I 



SECT. I. 



T is now generally believed that the bituminous fubftances are not of mineral origin, 

 but that they have been formed from certain principles of fubftances belonging to the 

 organized kingdoms of Nature, which, after the lofs of animal and vegetable life, have fuffer- 

 ed confiderable changes by long contaft and union with mineral bodies. 



Thefe changes have been however fo confiderable, that the bitumens can no longer be re- 

 ferred to their firft origin, and they are therefore regarded by general confcnt as forming 

 part of the prefent mineral fyftem. 



• Read before the Linmean Society in June and July 179T. 



Vol, 11 — August 1798. ' Dd The 



