146 Dr. Akin's further Statements concerning 



indicated in language which I am altogether at a loss to construe. 

 Why " a proposed experimental demonstration of a point which 

 can only be decided by experiment/' should be necessarily " hope- 

 less " and " absurd/' because it is a " proposition/' i. e. a " pro- 

 posal/' it is really beyond ordinary reasoning powers to compre- 

 hend. I proposed three experiments by which I hoped, with more 

 or less certainty, to produce calcescence ; to say that I advanced 

 the mere " proposition " as equivalent to a demonstration of fact, 

 which is the only intelligible meaning I can assign to Prof. Tyn- 

 dall's words, is one of those vagaries of language peculiar to Prof. 

 Tyndall, and to other examples of which I shall yet have to advert. 

 Prof. Tyndall further criticises that of my proposed experi- 

 ments which he allows to be " rational " — the " only rational " 

 one among the three which I had suggested. I had hoped to 

 produce incandescence by means of a concentrated solar beam 

 sifted by a diaphragm of monochromatic red glass. " It is need- 

 less to remark," Prof. Tyndall says, " that, even had this experi- 

 ment succeeded, the question would have still remained unsolved ; 

 for a sheet of glass, which permits the most powerful rays of the 

 visible spectrum to pass through it, could not be called a ' proper 

 absorbent.' " I assert that red glass is a " proper absorbent." 

 I had proposed to myself a double object : — first, to produce in- 

 candescence by invisible Herschellic rays ; and secondly, to pro- 

 duce visible or Newtonic rays of a given refrangibility in incan- 

 descent substances, by the incidence of other visible rays of 

 inferior refrangibility — for instance, green rays by the incidence 

 of red rays*. This latter experiment, although less striking to 

 the eye than the transmutation of invisible Herschellic rays into 

 visible or Newtonic rays, would yet possess quite as great an 

 importance as the first experiment in a theoretical point of view ; 

 and for its realization red glass is a " proper absorbent." I was, 

 however, at no loss regarding absorbents capable of separating 

 the invisible Herschellic rays wholly from the visible or Newtonic 

 rays. From Melloni's memoirs, I knew that black glass of a 

 certain variety is an absorbent of this kind ; and from Prof. 

 Tyndall's work on ' Heat ' — many months before he " performed 

 the thankless task of communicating to me his ideas " by word 

 of mouth — I had learned that a solution of iodine in bisulphide 

 of carbon possessed similar qualities. Ever since 1863 I endea- 

 voured to procure black glass, in which, till lately, I did not 

 succeed; and in the spring of 1863 (that is, many months before 

 Prof. Tyndall's thoughts had turned, upon his own showing, to 

 the subject of ray- transmutation as exemplified in the lime-light, 



* Prof. Tyndall only shows how little he understands the subject even 

 now, when he says that the " real problem " may be broadly stated thus : 

 — " To raise the refrangibility of invisible rays of long period, so as to con- 

 vert them into visible rays." 



