1869.] Physics. 465 



parts of the opal which emit red, yellow, green, or blue light are 

 opaque to light of the same refraiigibility which they emit. The 

 appearances of some opals examined in this way are very striking. 

 In the next number of the ' Quarterly Journal of Science ' Mr. 

 Crookes will describe these and other kindred phenomena more 

 fully, but we may briefly allude to the following curious spectra 

 which have been observed : — 



Some opals show a simple black band in the red, which when in 

 focus has a spiral structure. Examined with both eyes, it bears 

 a resemblance to a twisted column. 



In another opal, which gives an irregular line in the orange, the 

 spiral structure is shown in a marked manner, the different depths 

 and distances standing well out ; upon carrying the opal slowly 

 from left to right, the line is seen to revolve and roll o\'er, altering 

 its shape and its position in the spectrum. It is not easy to retain 

 the conviction that one is looking merely at an absorption baud in 

 the spectrum, and not at a solid body possessing dimensions, and 

 in actual motion. Other opals show an absorption band travelling 

 along tlie spectrum, almost from one end to the other, as the opal is 

 moved sideways. 



M. Soret, in consequence of Professor Tyndall's note " On the 

 Clouds," has examined, by means of the polariscope, the beautiful 

 blue colour exhibited by many parts of the Lake of Geneva, and he 

 states that this colour is due to the presence of solid particles in 

 the water, of the same specific gravity as that fluid ; but he does 

 not say what these particles are, nor what size or shape they have ; 

 promising, however, further researches. 



In order to answer the often-doubted fact of the decomposition 

 of carbonic acid under the influence of light, and the separation of 

 oxygen by the leaves of plants, Boussingault has introduced into 

 mixtures of carbonic acid gas and hydrogen, and the former gas 

 and nitrogen, first a clean stick of phosphorus; as long as no 

 oxygen is present, this element docs not undergo slow combustion, 

 thereby giving off vapours ; but as soon as a green leaf of any 

 plant was carefully brought into the gaseous mixtures standing 

 over mercury, the slow combustion of the phosphorus began, owing 

 to the decomposition of the carbonic acid and formation of oxygen ; 

 this action takes place also in diffused daylight, but not during 

 twilight ; leaves wherein the chlorophyl is not fully developed do 

 not act in this manner. 



It is a well-known fact that, even independent of the effects 

 of rain and wind, glass, even of good quality, is affected by 

 sunlight. The late Dr. Faraday made some observations con- 

 cerning this subject, and found that violet-coloured glass became 

 deeper and more intensely coloured than it originally was, after 



