W. Ferrel — Lata of Thermal Radiation. 3 



which have this peculiarity, as also certain modern Cycads of the 

 genus Zamia, which Professor Penhallow has kindly pointed 

 out to me. The present plant would seem to be a form of 

 Cordaitece, tending to JVceggerathia, which many paleobotan- 

 ists believe to have been a gymnospermous genus allied to 

 Cordaites. The affinities, however, so far as can be judged, 

 are nearer to the latter ; and following the example of Grand 

 Eury in his nomenclature of the genera, I would propose the 

 name Dictyo-cordaites for the present genus, and the specific 

 name Lacoi, in honor of its discoverer. 



It is apparent that this specimen combines the fructification 

 of the Cordaitece with leaves akin to those of JVceggerathia, 

 thus connecting two groups of paleozoic plants, both of which 

 are now considered as allied to. Cycadew and Taxinece, and I 

 entertain the hope that when it is fully studied and brought 

 into comparison with other specimens in my collections, or 

 which have been figured and described by other paleobotanists, 

 it will throw additional light on a great number of Paleozoic 

 Canadian leaves, fruits and stems, now designated as Cordaites, 

 JVosggerathia, Psygmophylhcm, Gingkophyllum, Sternbergia, 

 Lepidoxylon, Saportea, etc. ; and which have been waiting for 

 some specimen thus complete to bring them into harmony 

 with each other. 



I hope to be able to bring the whole of this material, which 

 will necessitate some change in the nomenclature of some of 

 my own species, under the notice of geologists at the approach- 

 ing meeting of the American Association. 



I may add that the oldest true Cordaites known to me is C. 

 Robbii of the Middle Devonian, which is said to have also been 

 found in the Silurian. C. angustifolia of the Lower Devonian 

 is a somewhat uncertain species. Plants of the genus Ncegge- 

 raihia are known in the Upper Devonian. 



Art. II. — The Law of Thermal Radiation • by 

 William Ferrel. 



1. It is well known that as the temperature of a body is 

 increased, the intensity of its thermal radiation is likewise in- 

 creased, but with regard to the law of increase, or relation be- 

 tween the intensity of the radiation and the temperature of the 

 body, there is still considerable uncertainty even within the 

 temperature range of experiment and observation. The two 

 principal formulae expressing this relation are that of Dulong 

 and Petit, given more than seventy years ago,* and that of Ste- 



* Journal de l'Ecole Polytechnique, xi, 234-294. 



