44 Prof. J. Dewar. The Boiling Point of Liquid Hydrogen, 



Thus, if we suppose that Mr. Forbes measured the total light within 

 a circular area 5° in diameter, which seems a fair supposition,* the 

 two measures of total brightness agree. 



On the same supposition, the value of B (R 2 - 1) in 1898 would be 

 1*3 full moon, and the total brightness of the corona would appear as 

 1-1 + 1-3 = 2-4 full moon. 



Summary. 



(a.) The brightness of the corona of 1898 at a point distant r from 

 the sun's centre expressed in solar radii may be approximately repre- 

 sented by the formula 



brightness = Ar~ fl + B, 



where A and B are constants. 



(b.) The first term may be considered as corona proper, while B may 

 be taken as representing the constant illumination of the sky, or glare. 

 In 1898 the value of B was 2~ 6 ' i = 0*012 moon, taking the brightness 

 of the moon as 0'02 candle at 1 foot. 



(c.) The constant A varies with the radius along which measures are 

 made. In 1898 it varied from 2°'° moon to 2 1 ' 9 moon, the mean being 

 2 115 moon or 2*2 moon. 



(d.) The same formula will fairly represent the 1893 corona, the 

 mean value of A being 2° 23 = 1*2, and the value of B 2~ 7 ' 8 = 0*0046. 



(<?.) The total brightness of the corona depends on the area of sky 

 included. If a circular area 5° in diameter be included, the total 

 brightness of the 1893 corona may be taken as 1*1 full moon, agreeing 

 with the visual measures made, and that of 1898. on the same supposi- 

 tion, would be about 2*4 full moon. 



" The Boiling Point of Liquid Hydrogen, determined by Hydrogen 

 and Helium Gas Thermometers." By James Dewar, M.A., 

 LL.D., F. II. S., Professor of Chemistry at the Royal Institution, 

 and Jacksonian Professor, University of Cambridge. Re- 

 ceived January 8, — Read February 7, 1901. 



In a former paper f it was shown that a platinum-resistance thermo- 

 meter gave for the boiling point of hydrogen - 238°*4 C, or 34°*6 



* The dimensions of the box are not given, either here or in the previous paper 

 to which we are referred ; but on p. 369 of the ' Philosophical Transactions, 

 A, 1889, there is a diagram of the box, from which it would appear that the angular 

 aperture was not greater than 12°, judging by outside measurements. 



f "On the Boiling Point of Liquid Hydrogen under Reduced Pressure," ' Roy. 

 ■Soc. Proc.,' 1898 (vol. 64, p. 227). 



