the History of Calcescence. 145 



tographic impression produced by an oxyhydrogen-flanie in the 

 space of 20 seconds was found to be very faint. The expres- 

 sions of ' ( poor " and " rich/' however, are well known to be of 

 comparative value only; and Prof. Tyndall is well aware that 

 I grounded my conclusions regarding the origin of lime-light, 

 not upon the poverty or richness of the oxyhydrogen-flame in 

 Ritteric rays in its natural state, but upon the comparative abun- 

 dance of Newtonic and Ritteric rays emitted by the oxyhydrogen- 

 flame and by the lime-light. For the purpose of proving this, I 

 need refer only to the note which Prof. Tyndall certainly had be- 

 fore him in writing his late paper. I there expressly state that 

 I relied in my reasonings upon the probable poverty of the oxy- 

 hydrogen-flame in Ritteric rays, as compared with lime-light, 

 which was later corroborated by the experiments of Dr. Miller*. 



Not content, however, with showing up imaginary defects in 

 my reasoning on the origin of lime-light, Prof. Tyndall attempts 

 to prove also the existence of " radical vices " in my experi- 

 mental suggestion for the production of calcescence. What I 

 " imagine/' he says, in my first proposal, " is plain enough," 

 viz. "that the whole heat of the flame [the italics are Prof. 

 TyndalPs] is radiated against one mirror and condensed by the 

 other." And he continues, " It is not the practical difficulties, 

 which Dr. Akin himself discerns, that I am now speaking of; 

 it is the radical vice of the conception that a purely gaseous 

 flame, placed in the focus of a mirror, however large, could pos- 

 sibly generate a temperature ( approximately equal to that of the 

 flame itself/ in the focus of another mirror." What Prof. 

 Tyndall means by the phrase, " the whole heat of the flame is 

 radiated against one mirror," I am at a loss to understand ; but 

 the point on which he impugns the validity of my reasoning is 

 stated in the latter part of the sentence quoted, and which is 

 plain enough. Now, I would ask any one, knowing the labours 

 of Fourier and others, what would be the consequence, according 

 to theory, if at one of the foci of a vacuous ellipsoidal envelope of 

 proper form and perfect reflecting power an oxyhydrogen-flame 

 were placed, and at the other focus a piece of platinum, for in- 

 stance ? Which is the " amusing " supposition ? is it to suppose 

 that the platinum would ultimately attain to the temperature of 

 the flame ? or is it to suppose, as Prof. Tyndall does, that this 

 latter assumption is "absurd?" "As a proposed experimental 

 demonstration," Prof. Tyndall continues in his urbane language, 

 " of a point which can only be decided by experiment, Dr. Akin's 

 third proposition is, if possible, more hopelessly absurd than his 

 first." At whose door the absurdity lies in the first case, I have 

 already demonstrated ; but this second imputed (i absurdity " is 

 * See Phil. Mag. vol. xxviii. p. 556. 



Phil. Mag. S. 4. Vol. 29. No. 194. Feb. 1865. L 



