March 2, 1900.] 



SCIENCE. 



353 



municated to the Berlin Academy of Sciences 

 in May, 1899. This memoir, which seems to 

 be monumental in character, is remarkable in 

 that it verifies the Stefan-Boltzmann law, de- 

 rived from thermodynamic considerations, that 

 the total energy radiant from a black body is 

 proportional to the fourth power of the abso- 

 lute temperature, and the law of W. Wien con- 

 cerning the distribution of energy in the spec- 

 trum of a black body. 



Wien's conclusions are based upon certain 

 assumptions as to the number of radiant centers 

 (molecules) in unit volume and their velocity. 

 It is now known that the total energy radiated 

 from a black body and its distribution in the 

 spectrum depend only upon temperature and 

 are entirely independent of the physical prop- 

 erties of particular substances, so that it is 

 highly probable that" the law of total energy 

 and the law of its distribution in the spectrum 

 are capable of rigorous derivation from assump- 

 tions of axiomatic simplicity. 



The theoretical results of Stefan, Boltzmann 

 and Wien, now verified by Planck, may, there- 

 fore, eventually appear to be independent of 

 the highly specialized character of the assump- 

 tions upon which they are based. When this 

 stage of the science is reached, these laws of radi- 

 ation will no longer appeal to experiment for ver- 

 ification, but they will take their place among 

 numerous other established laws as instruments 

 for the interpretation of experimental results. 



Physicists ought to drop the term radiant en- 

 ergy and retain the older and better term radiant 

 heat, inasmuch as the energy of radiation is heat 

 in the same sense that molecular energy is heat. 

 Both types of energy are subject to the first and 

 second laws of thermodynamics ; both types 

 give rise to the entropy function, and Max- 

 well's law of molecular velocity distribution is 

 strictly analogous to Wien's law of the distribu- 

 tion of energy in the spectrum. 



THERMAL CONDUCTIVITY. 



Heat measurements are among the most 

 inaccurate of physical measurements and the 

 measurement of thermal conductivity is perhaps 

 the most inaccurate of the measurements in 

 heat. Professor Kohlrausch {Drude's Annalen, 

 January, 1900) proposes a method for measur- 



ing thermal conductivity which depends upon 

 the final permanent distribution of temperature 

 in a conductor carrying electric current, heat 

 being allowed to flow out of the conductor only 

 at the points where current enters and leaves it. 

 Under these conditions a remarkably simple re- 

 lation subsists between the temperature at a 

 point, the electric potential at a point, and 

 the ratio of electrical to thermal conductivity. 

 The method depends only upon measurements 

 of temperature, of electrical potential, and of 

 electrical conductivity. W. S. F. 



ENGINEERING NOTES. 



A NUMBER of European nations are now 

 adopting the Gruson chilled iron shield for their 

 land defences and the success of the invention 

 is so well-assured, it is said, that the Messrs. 

 Krupp, some time since, bought the Gruson- 

 werke and have developed the invention to a 

 state of considerable perfection. The Gruson 

 armor-turrets are thought to be practically in- 

 vulnerable ; their flatly curved tops deflect- 

 ing shot and shell and their adamantine chilled 

 surfaces and their great thickness making them 

 impenetrable to direct impact of the heaviest 

 shot. It is proposed to endeavor to intro- 

 duce this device into the United States, where 

 it is thought that it may be made even more 

 successful, since our chilling irons are found to 

 be superior to those of any other country. The 

 turrets are usually of from 50 to 100 tons weight 

 and are built up of great staves and segments, 

 ten or fifteen of which constitute the low, wide, 

 circular, covered box which constitutes the tur- 

 ret and protects the guns. The top is usually 

 made of two semi- circular halves. In their man- 

 ufacture, the quality of iron employed is presum- 

 ably that found to make the best car wheels and 

 one of peculiar strength and toughness, as well 

 as of intensely hard chilling property. A Oru- 

 sonwork is to be established at Chester, Pa., by 

 New York and Philadelphia capitalists. 



The success of the submarine craft which 

 have been recently produced in the United 

 States and in France is stimulating other 

 nations, and an authority among English tech- 

 nical journals — Industries and Iron — says : "In 

 spite of the derision with which they have been 



