A PLATINUM SURFACE AT HIGH TEMPERATURES. 509 



Part II. A BOLOMETRIC STUDY OF THE LAW OF THERMAL RADIATIQN. 



The relative merits of the laws of DULONG and PETIT, and STEFAN, have formed 

 the subject of a number of researches. It is sufficient to recall the experimental 

 work of SCHLEIERMACHER * and ScHNEBELi,t which appeared in 1884, and the 

 theoretical discussion of the subject by FERREL^ some five years later. In 1888 

 Professor WEBER proposed a new expression connecting the thermal radiation with 

 the absolute temperature of the radiator and the enclosure. This expression, as 

 he showed, accounted satisfactorily for the greater part of the experimental results 

 then available. It was with the hope that some additional data with regard to the 

 total radiation at higher temperatures might help the elucidation of the problem that 

 the present experiments were carried out. 



Most of the apparatus employed has been already described in Part I. For the 

 experiment we are now about to describe the radiating wire was placed in a metal 

 enclosure kept at a constant temperature by a water circulation. A number of 

 diaphragms, also provided with water circulations, shielded the bolometer from all 

 external radiation. 



The bolometer used was of a somewhat special construction. Many weeks were 

 spent in preliminary experiments before the design of the instrument was finally 

 fixed. It is with a desire to spare this loss of time to others that a somewhat full 

 description of the bolometer is given here.|| 



The instrument is symmetrical throughout, one side containing the active platinum 

 film which is exposed to the radiation, the other side an exactly similar film forming 

 the second arm of the bridge. For the platinum silver foil from which these films 

 were made I am indebted to the kindness of Mr. J. S. SELLON, of Messrs. JOHNSON, 

 MATTHEY and Co. 



A thin sheet of platinum was welded to a thick sheet of silver, the two sheets, 

 protected on each side by a copper plate, were then rolled together and a leaf was 

 thus obtained, of sufficient thickness to be quite easily handled. This I cut on the 

 dividing engine into the shape of a grid (as shown in fig. 8, F). It was then placed 

 in the holder (fig. 8, A), and the silver dissolved in dilute nitric acid. The film of 

 platinum left was found to have a thickness of '00011 centim., or about twice the 

 wave-length of sodium light. The active surface has the appearance of a grating, 

 consisting of fifteen bars, 3 millims. wide, with a space of 1 millim. between each 

 bar. The span from the upper to the lower edge of the carrier is 56 millims. The 



* ' Wied. Ann.,' vol. 26, p. 287. 

 f 'Wied. Ann.,' vol. 22, p. 430, 1884. 

 J ' American Journal of Science,' vol. 38, p. 3, 1889. 

 'Berichte der Preussischen Akademie,' 1888, p. 933. 



|| Those interested in this subject will do well to refer to an article by LUMMER -and KOELBAUM 

 (' Wied. Ann.,' vol. 46, p. 204, 18D2 ; or, ' The Electrician,' vol. 34, pp. 168, 192, 1894). 



