I 



Prof. Airy's Reply to Sir David Brewster. 4J9 



oxide of iodine; and if such be the case it will easily account 

 for the absence of oxygen in the iodic aether, though formed 

 by the agency of nitric acid. It is probable that nitric aether 

 is formed first, and that from its decomposition the new com- 

 pound containing iodine results. It seems to me, therefore, 

 most likely that the aether described in this paper is a com- 

 pound of iodine and aBtherine (4 C + 4H), belonging probably 

 to the same class of compounds as the solid iodide of Mr. Fa- 

 raday. Indeed in one experiment, instead of the aether sub- 

 siding as I expected, I obtained a group of large crystals of 

 the solid iodide of carbo-hydrogen *. 

 Portobello, April 22, 1833. 



LXIX. Remarks on Sir David Brewster's Paper " On the Ab- 

 sorption of Specific Kays, $fc" By G. B. Airy, Esq, M.A. 

 Plumian Professor of Astronomy and Experimental Philo- 

 sophy in the University of Cambridge. In a Letter to Sir 

 David Brewster, K.H. LL.D. F.R.S. S>c. $c. 



My dear Sir David, 



N commenting upon your paper in the last Number of the 

 Philosophical Magazine, I cannot but feel that I am un- 

 dertaking an invidious task. That you will misinterpret my 

 motives or feelings I am not afraid ; but to others it may ap- 

 pear presumptuous in me to criticize the remarks of one whom 

 I revere as the author of nearly all our experimental know- 

 ledge in the most important parts of optics. But science is 

 public property: it is the right of all, and may be the duty of 

 some, to expose what they conceive to be erroneous; and the 

 obligation is at least not lessened when such seeming error is 

 backed by the highest scientific character. 



I commence with your remarks on the test of theory. " The 

 power of a theory to explain and predict facts is by no means 

 a test of its truth ; and in support of this observation we have 

 only to appeal to the Newtonian Theory of Fits, and to Biot's 

 beautiful and profound Theory of the Oscillation of Luminous 

 Molecules." I must surely have misinterpreted this sentence. 

 That theories essentially and fundamentally different can apply 

 equally to the explanation of phaenomena embracing so many 

 classes as the phaenomena of optics, is, I conceive, ouite im- 

 possible. What test, then, can there be for the truth ot a theory 

 but the power which it gives us of calculating old observations 



* The colourless transparent prismatic crystals described in this paper 

 as Faraday's iodide, differ from that compound, as generally described, id 

 being slightly soluble in water, from which they may be again volatilized in 

 beautiful prisms by a verv gentle heat. 



3 H 2 



