C. GENERAL PHYSICS. 137 



becomes strongly heated before it undergoes combustion it 

 a^ain becomes luminous. 



From these observations it may be concluded that the 

 non-luminosity of a gas flame is not caused by a dilution of 

 the gas, this dilution being in fact increased by heating the 

 mixture. The only cause is the cooling of the interior 

 of the flame. This is further proved by the fact that it 

 is most difficult to get a non-luminous flame from a mixt- 

 ure of coal-gas and oxygen, showing that neither rapid ox- 

 idation nor dilution produces the non-luminosity. 21 A, 

 July, 603. 



FLAME OF BURNING GLYCERINE. 



According to Godeffroy, glycerine burns with a steady blue 

 non-luminous flame, without diffusing any odor or leaving any 

 residue. 17 A, June 1, 84. 



A PERFECTLY MONOCHROMATIC SODIUM FLAME. 



Laurent recommends the following simple method for 

 rendering the light proceeding from a soda flame, which it is 

 desired to use for saccharometric and similar work, perfectly 

 monochromatic. Between the flame and the polarizer he in- 

 terposes a thin lamina of potassium bichromate, which pos- 

 sesses the property of absorbing the violet, blue, and green 

 rays contained in the sodium flame, the presence of which 

 impairs the accuracy of observations involving a comparison 

 or determination of the equality of tints. 2 C\ III., 1875, G2. 



AN APPARATUS FOR ILLUSTRATING THE MECHANICAL EFFECTS 



OF LIGHT. 



Dr. William Crookes is the first to illustrate by experi- 

 ment the production of direct mechanical effects by the ac- 

 tion of luminous rays. The apparatus, with the aid of which 

 the demonstration was made, and to which its designer gives 

 the name of the radiometer, is described as follows: The ra- 

 diometer consists of four small pith disks, fixed at the ex- 

 tremities of two crossed arms of straw, balanced on a pivot 

 at the point where the straws intersect, so that they may free- 

 ly spin round. These pith disks are white on one side and 

 blackened with lampblack on the other, and the entire ar- 

 rangement is inclosed in a crlass bulb from which the air has 



