Chemistry and Physics. 55 



burner. Mr. St. John shows that the method of coating an incan- 

 descent platinum wire with the various oxides, the light-emissions 

 of which are to be examined, leads to inaccurate results at high 

 temperatures. He was led to examine also the mooted question 

 whether the light of the above burners is rich in phosphorescent 

 and fluorescent rays. Various methods which he employed, how- 

 ever, showed that these rays were very slight. The most import- 

 ant part of the research relates to the use of what may be termed 

 the oven method of testing the light-emission. In this method 

 the two bodies whose emissive power is to be determined are 

 placed in an oven the temperature of which was raised 1100°- 

 1200° C, a range of temperature which was determined by the 

 pyrometer of Holborn and Wien. Two pieces of platinum foil 

 were suspended side by side in this oven and the observation of 

 their incandescence was made through a suitably placed window, 

 by means of a Glan's spectrophotometer. A Triplex lamp served 

 as the standard light. When one piece of foil was coated with 

 the oxides it appeared brighter than the uncoated piece. If one 

 piece of foil was inclined to the wall of the oven so that the 

 reflected rays were sent through the window the two pieces of 

 foil could be made to appear of the same intensity. The sum of 

 the direct and reflected light is then for both pieces of foil equal. 

 The uncoated piece must reflect just as much more light than the 

 coated as it is deficient in the amount of direct light it can trans- 

 mit. This is in accordance with Kirchhoff's law that in a heated 

 space a bundle of rays made up of direct and reflected rays from 

 a surface shows the same peculiarities that a bundle of rays from 

 a dark hot body would show. Mr. St. John utilized this idea by 

 bringing a porcelain cylinder into the neighborhood of the pieces 

 of foil. The bare platinum could then be quickly distinguished 

 from the surrounding hot walls and appeared darker than the 

 coated platinum. As soon as the rod took the temperature of the 

 oven the field of view appeared uniformly bright. 



The spectrophotometer was adjusted for light of a determined 

 wave length, and the image of one of the two bodies was thrown 

 on the slit. The porcelain rod was then placed in the furnace, the 

 light was observed ; by means of a piece of plane parallel glass, 

 the image of the other body was brought into the field. The 

 porcelain rod acts to cut off the reflected light. Mr. St. John 

 gives the following table : 



X= 0-515 







Magnesium' 



Zirkonium 



Erbium 



Lanthanum 





Platinum. 



oxide. 



oxide. 



oxide. 



oxide. 



ccording to the electrical j 



1 











method of glowing plati- 



>■ 1-00 



1-26 



1-81 



1-70 



1-88 



According to the oven 

 method _ _• __ 



1-00 3 81 4-04 3-35 2'21 



The inaccuracy of the electrical method is clearly shown. The 

 results of Mr. St. John show that the Kirchhoff's law leads to a 

 practical method of measuring in an oven the light-emission of 

 bodies at high temperatures. — Ann. der Physik und Chemie, 

 No. 11, 1895, pp. 433-450. j. t. 



