W. /v/7v7— Weber'* Law of Thermal Radiation. 137 



ment, as they were without exception psendomorphe and the 

 faces had lost to a greater or less extent the high polish and 

 luster of ordinary crystal faces. Many of the measurements 

 were therefore made from very indistinct reflections, being 

 mere " schimmer " measurements and in such cases the angles 

 vary at times more than one degree from the calculated. 

 Among all the measurements the agreement between the 

 measured and calculated angles is very satisfactory, considering 

 the nature of the material, while the zonal relation of the faces, 

 as observed on the goniometer, was throughout very perfect. 

 From the following statement a rood idea of the frequency of 

 the faces can be obtained : leaving out of consideration two 

 stalactites, No. 2, terminated by only three faces, and No. 3, 

 by only two, we find that the faces, o, 001, b, 010 and p y 111 

 were identified on all of the remaining eleven crystals ; M, llO 

 on ten; ?//, 110 on nine; e, <>:H and /\ la2 on eight; d, 02 L on 

 n ; x, 132 on six; //, TIL and c>, ~\\2 on four; n, III and A, 

 150 »>n three; /'. 310, <j. 312, z, §12, tt, 152 and />, 131 on two 

 and /, L*0, v. 1:2, f §01, P, HI, r, 10, 12, 11 and w, 5, 151 on 

 only one crystal. Owing to the pseudomorphous nature of the 

 mineral we cannot give any statement of the optical properties 

 of the lansfordite. The cleayage of the lansfordite, which 

 Dr. Genth mentioned in the original article as being very good 

 is probably basal, which we infer from the fact that the 

 partially altered crystal No. 11 broke near its base, where it 

 was wholy altered, parallel to the basal plane and lower down 

 where the mineral was only partially altered the decomposition 

 seemed to be advancing along planes, probably cleavage, 

 parallel to the base. 



Chem. Laboratory, 111 S. 10th St., Philadelphia. 



Mineralogical Laboratory, Sheffield Scientific School, Dec. 10th, 1889. 



Art. XX. — Weber* s Law of Thermal Radiation', by 

 William Ferrel. 



1. In a previous paper on the Law of Thermal Radiation,* 

 which will be referred to in what follows as paper A, the laws 

 of Dulong and Petit and of Stefan were examined, and it was 

 shown from comparisons with the results of experiment that 

 neither of these laws holds generally for all temperatures of 

 the radiating body, but that either, and especially the latter, 

 holds through a considerable range of the ordinary tempera- 

 tures of experiment and observation, and that for higher or 

 lower temperatures, a change in the values of the constants A is 



* Thi9 Journal, xxxviii, July, 1889. 



