132 ANNUAL OP SCIENTIFIC DISCOVERY. 



twice its visible length, and that the invisible portion of it contains 

 8 or 9 times more heat than the visible length. 



Dr. Frankhind's experiments deal with the effects upon light 

 of pressure made to act upon luminous sources. After pointing 

 out that the more the j^ressure of the outside air is increased, the 

 more luminous will it make the flame of the common spirit-lamp, 

 he showed how to make the oxyhydrogen flame luminous ; as the 

 flame continued to burn within a strong iron cylinder, under a 

 constantly increasing pressure, produced by the the tension of the 

 gaseous products of combustion, the light grew brighter, till, at a 

 pressure of about 150 pounds to the square inch, the light was 

 nearly as bright as that of a common candle. He showed that 

 external atmospheric pressure increases the luminosity of flames. 

 Common gas gives out a little more light when the barometer 

 rises, and decreases in luminosity as it fiiUs. Some flames, as 

 that of burning metallic arsenic in oxygen, are very brilliant, 

 though they contain no solid j^articles, and their brightness in- 

 creases with the density of the gases or vapors burning, or pro- 

 duced by the combustion. He also proved that the spark from 

 the secondary wire of an induction coil is brighter in jjroportion 

 to the density of the gas by which the spark is surrounded. He 

 gives the following relative densities of gases and vapors : hydro- 

 gen, 1 ; ammonia, 8^ ; water, 9 ; air, 14i ; oxygen, 16 ; hydro- 

 chloric acid, 18| ; carbonic anhydride, 22 ; sulphurous anhydride, 

 32 ; chlorine, 35^ ; phosphorus, 62 ; mercur}', 100 ; phosphoric 

 anhydride, 142; arsenic, 150; arsenious anhydride, 198. — Me- 

 chanics* Magazine. 



CHEMICAL RAYS OF THE SUN. 



Most of the waves travelling to the earth from the sun have no 

 power to act upon the organs of vision, but convey only the sensa- 

 tion of heat. About eight-ninths of the rays reaching the earth 

 from the sun are too long and slow to make their presence known 

 to our eyes, and these waves are those whose powers and quali- 

 ties have been largely examined and investigated by Dr. Tyndall. 

 But there are other waves at tlie other side of the solar spectrum, 

 too short in length and too rapid in motion to excite the organs of 

 vision ; neither are they sufliciently powerful to make their pres- 

 ence known in the shape of heat. 



These excessively short waves coming to us from the sun have 

 been thoroughly examined by Prof. Stokes, Secretary of the Royal 

 Societ3^ 



Some of these chemical rays are visible to the eye, and some of 

 them, as already stated, are invisible, and extend beyond the vio- 

 let end of the spectrum. 



If a slice of light from the sun or electric lamp, after passing 

 through a slit in an opaque screen, be received upon a glass prism, 

 and tlien be brought to a focus upon a white screen by means of a 

 lens, the prism will " unroll,'' so to speak, the line otNvhite light, 

 and spread out all its rays upon the screon in the order of their 



