144: ANNUAL OF SCIENTIFIC DISCOVERT. 



nitrotren gas to enter, and, on passing a cone of sunlight from a 

 lens throiigli the long axis of the tube, he was surprised to see a 

 cloud forming, because of chemical decomposition set up. At 

 first he thouLCiit tiiat the resinous cement fasteninfc the brass fer- 

 ulos to the ends of the glass tube had liberated some volatile 

 hydro-carbon, such as turpentine, and so introduced foreign ele- 

 ments into the experiment. He consequently substituted for the 

 tube a long, narrow measuring glass, with a foot, and a wide 

 circular mouth, ground Hat. Over the Hat moutli he cemented a 

 flat plate of glass, by means of a mixture of oil, wax, and tallow. 

 A glass stopcock was let into the side of the tube. Still, with the 

 dry hydrogen and nitrogen, he obtaineil cloudy decomposiiiou 

 under the action of sunlight. This led him to question the method 

 employed to dry tiie gases, which was by passing them through 

 powdered glass wetted with sulphuric acid. When the chloride 

 of calcium and other methods of drying girses were tried, no 

 clouds were formed by the sunlight, so at last he came to the con- 

 clusion that the source of error lay in a trace of sulphurous-acid 

 gas, taken up by the hydrogen antl the nitrogen from the sulphuric 

 acid. The latter acid employed by him was absolutely pure, and 

 contained no trace of arsenic from the use of impure sulphur 

 in its manufacture. In the remainder of his ])aper he explained 

 the exact nature of the chemical reactions which took place in his 

 tube, which reactions he, like Dr. Tyndall, ascribed to a motion 

 of separation set up between the atoms of each molecule, by the 

 short blue and violet waves of the solar spectrum. Dr. J. 11. 

 Gladstone, F.R.S., spoke highly of the philosophical character of 

 the paper, and said that such researches promised ere long to 

 explain what action sunlight has upon gases and vapors which are 

 often present as impurities in the atmosphere. 



ROCK-SALT PRISMS. 



Mv. C. Brooke, F.R.S., said at the meeting of the British Asso- 

 ciation that, in his attempts to grind and polish rock-salt prisms in 

 planes liOt i)arallel to the lines of cleavage of the crystal, lie found 

 that the jxirtly ground prisms usually cracked and broke at the 

 thinnest end. lie ami ^Ir. Bnnvning, the optician, consequently 

 tried the plan of slowly heating the rock salt buried in sand in a 

 tin vessel, and then pcrmitteil the whole to cool very slowl}. 

 After this annealing had been performed, it was possible to grind 

 the rock-salt into good prisms. 



TWILIGHT, AND THE UNEQUAL EFFECT OF LIGHT IN PHOTOG- 



RAPIIY. 



1. Cause of the unequal Visibility of the Colors in the Twilight. — 

 Tiic vitiijiiity of the i.-olors of a ])icuue imply ethereal waves, ex- 

 ci'eil bv Hieir \ ibrations, and which tran>mit themselves to the 

 retina. But these vibrations of the colors want themselves to be 



