1 50 Dr. Joule on the Histonj of the 



together. Spherules remarkably round, and, in section, very 

 dark and brilliant, are plentifully distributed through the mass 

 of the stone, and are associated with others of a lighter hue. 

 The aerolite is very compact and takes a good polish. Its spe- 

 cific gravity is 3*63. It contains Troilite in about an equal 

 proportion in bulk to the iron, and it is similarly disseminated. In 

 the microscope some of the spherules are seen to consist of the 

 grey mineral, with a fan-like structure radiating from an edge or a 

 point external to the spherule, apparently belonging to one of 

 the oblique systems. There are also the usual kinds of spherule, 

 in which the clear olivine-like mineral is seen forming a sort of 

 breccia, and engaged in a flocculent aggregate of confusedly 

 crystalline matter, from which a ray of polarized light emerges 

 without any definite plane of polarization. Similar ingredients 

 fill the interspherular space, and the aspect of the aerolite in the 

 microscope is that of a pretty uniform mixture of this flocculent 

 aggregate and fragmentary but clear crystals. There are like- 

 wise a few crystals or broad clear bars of crystal, unlike the 

 ordinary olivine in their habit, but with their plane of polarization 

 parallel or perpendicular to the direction of the bar. It is not 

 likely to be augite seen in a section parallel to the plane 10 

 (and perpendicular to the axis of symmetry) ; and it is difficult 

 to resist suggesting the probability of its being enstatite, cut pro- 

 bably nearly parallel to one of its planes of cleavage. It is not 

 a rare ingredient of aerolites. 



XVII. Note on the History of the Dynamical Theory of Heat. 

 By James P. Jottle, LL.D., F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen,, 



SOME observations in Professor TyndalPs " Notes on Scien- 

 tific History " call for an early notice on my part. After 

 the perusal of this article, I freely admit that I erroneously took 

 the degrees in the fifth column of Seguin's Table for thermal 

 units in kilogrammes. I regret this oversight the more particu- 

 larly, as it seems to have misled others who have since written on 

 the subject. But I must still express my conviction that in the 

 statement of his hypothesis Seguin anticipated Mayer. To 

 prove this, I will give the following extracts from the Chemins 

 de Fer : — 



" il me parait plus naturel de supposer qu'une certaine 



quantite decalorique disparait dans Facte memede la production 

 de la force ou puissance mecanique, et reciproquement ; et que 

 les deux phenomenes sont lies entre eux par des conditions qui 

 leur assignent des relations invariables. II resulterait, comme 



