124 Prof. C. Bohn on Negative Fluorescence 



sibly a combustion, accompanied, like the slow oxidation of 

 phosphorus, with evolution of light. The gradual extinction of 

 the light would, upon such a view, have to be regarded as an 

 exhaustion of the combustible material; and in the interval of 

 repose needed for the reacquisition of the phosphorescent power, 

 a new supply of this material might collect at a low temperature 

 upon the surface, either by deposition from the air or from the 

 interior of the mass of fluor-spar. I do not, however, wish to be 

 understood as putting this forward as more than a possible 

 suggestion. 



22. On the other hand, I feel fully justified by the preceding 

 observations in denying the analogy between phosphorescence and 

 fluorescence. But if once the notion that the phosphorescence 

 of fluor-spar is a direct transformation of rays of one kind into 

 rays of greater refrangibility is given up or refuted, it is impos- 

 sible to regard the phenomena of incandescence produced by 

 Messrs. Akin and Tyndall as phenomena of the same kind as 

 fluorescence. 



Platinum wire becomes incandescent not only in a charcoal 

 fire, or by the action of a galvanic current, or in the flame of a 

 spirit-lamp, but also in the flame of hydrogen. The same ap- 

 plies to lime. Metals become incandescent and melt, and paper 

 burns, if a certain temperature is reached, however the heat 

 needed to cause the rise of temperature is produced. 



Such, in my opinion, is the significance of the experiments of 

 Messrs. Akin and Tyndall. The idea of negative fluorescence 

 in their case is out of the question. If water is heated to the 

 boiling-point, the properties of the hot water and of the steam 

 are exactly the same, whether the heat requisite for this purpose 

 was produced by the combustion of any substance whatever, or 

 by the discharge of electricity, or by the transformation of me- 

 chanical work. It is impossible for any one to detect in pure 

 warm water how anil where it has got its heat — whether from a 

 common fire or from the sun, in an iron boiler or in a porcelain 

 dish, or inside a volcano — whether it was previously hotter or 

 colder — whether it is condensed steam or melted ice. At a cer- 

 tain temperature chemical compounds are formed, and at another 

 they are decomposed ; the bulk and state of aggregation of bodies 

 depends upon their temperature, other circumstances being always 

 supposed constant ; temperature also determines the kind and 

 intensity of radiant heat, as well as incandescence and many 

 ether properties. But it is only the temperature which comes 

 into account, never the origin of the heat which has produced 

 this temperature. The manifold and great differences exhibited 

 by different kinds of radiant heat disappear as soon as the heat 

 passes from the dynamical into the statical condition (if I may be 



