I have only to add that Dr. Apjohn has admitted that I suggested the ex- 

 perimental methods to him, and never gave me the slightest intimatron that 

 they had previously occurred to him ; consequently whatever opinion may be 

 formed bv the reader on the subject, Dr. Apjohn has only to blame himself 

 for what has occurred. As to anything personal in Dr. Apjohn's numerous 

 and ungracious insinuations against me, I shall not notice them : throughout 

 the entire discussion it has been my wish to avoid all unbecoming language, 

 not only because such a practice is repugnant to my habits of thinking and 

 acting, but because I am convinced that by such means a good cause can 

 only be injured. 



With respect to the last subject alluded toby Dr. Apjohn, viz. his formula 

 for the specific heats of gases, I saw it for the first time on the 4th of No- 

 vember 1835 (in the Philosophical Magazine for that month), and, on meet- 

 ing him at the Academy on the 9th of November, told him that his formula 

 was erroneous. He smiled and shook his head. I may add that the moment 

 I saw his experimental depressions of the wet bulb's temperature in 

 common air and hydrogen gas, I perceived that his deductions were mon- 

 strously erroneous, on a principle that I did think must have been obvious to 

 any one who understood the spirit of the method. It is simply this:— If 

 a o-iven volume of one gas (in producing a certain evaporation) fall 20°, 

 while the same volume of another gas in producing the same (or a less de- 

 gree of) evaporation, falls 25°, it is evident that the specific heat of the for- 

 mer gas {estimated by volume) is greater than that of the latter gas. I 

 mentioned this principle to Dr. Apjohn on the occasion just alluded to, and 

 his answer was " Oh no ! depend upon it you are quite wrong; it has nothing 

 to do with the capacities estimated by volume." Under these circum- 

 stances therefore I sent the correct formula to the Philosophical Magazine 

 onthe 1 1th of November, and (I believe on the same day) communicated it 

 in a letter to Dr. Apjohn himself. To this letter, however, I received no 

 answer. 



The above is a plain statement of the facts as between Dr. Apjohn and 

 me ; but the reader of his account of the .same matter would necessarily be 

 led to believe that 1 was present at the meeting of the Chemical Section of 

 the British Association in August 1835, when he brought forward his erro- 

 neous formula (which was not the case); that I then pointed out the error, 

 which he immediately admitted, being thankful to me for the suggestion ; 

 and that I, notwithstanding his thus readily adopting my correction, for- 

 warded a paper on the subject to the Philosophical Magazine. If such had 

 been my course, I certainly might have deserved a share of that censure 

 which Dr. Apjohn has endeavoured to direct against me ; the reader, by the 

 facts and dates which I have supplied, but which are suppressed in Dr. Ap- 

 john's account, will now perceive that this, like his other charges against me, 

 IS wholly destitute of foundation. 



I am. Gentlemen, 



Your obliged and obedient Servant, 



H. Hudson. 



24, Stephens Green, 

 Wtk September 1836. 



