cepta note from Professor Lloyd, written more than a year after the various 

 papers had beoii communicated. In tliis note Professor Lloyd mentions :» 

 conversation with Dr. Apjohn reh\lina: to the determination of the specific 

 heats of gases by means of one of those methods of experimenting, stating 

 that it occurred on the evening on vvlich Dr. Apjohn read his ^rs< paper to 

 the Academy, a statement which Professor Lloyd has since distinctly reassert- 

 ed. With the highest respect, however, for Professor Lloyd, and in the most 

 ample manner admitting his truth and honour to be unquestionable, I can- 

 not but think he is mistaken respecting the evening on which Dr. Apjohn 

 made this communication to him, and that it probably took place in April 

 183.T when Dr. Apjohn read his second paper. After the lapse of more 

 than a year undoubtedly a mistake as to the evening on which a particular 

 conversation occurred is by no means impossible, and it appears to me in 

 the highest degree probable, from my inability to reconcile the statement 

 (or any other su])position) with the written evidence furnished by Dr. Apjohn 

 himself. Now this is Dr. Apjohn's only evidence, and how do the facts 

 stand on the other side? 



First. Dr. Apjohn made no allusions whatever in his first paper to insti- 

 tuting any experiments. He says, " I shall not at present refer to my own 

 observations, though I have of late amassed a considerable number on the 

 hygrometer and dew-point;" so that he had been (just before) amassing 

 observations to test his formula, which, if he had any intention, at the time, 

 of instituting experiments, would have been a very useless waste of time ; in- 

 deed he appears subsequently to have thought so, for in his second paper, 

 (speaking of observations oi this sort,) he says, " The observed depressions in 

 the table are, generally speaking, so small that a formula in itself incorrect 

 might, it must be admitted, yield results which would deviate from the ob- 

 served dew-points by quantities not exceeding the possible errors of observa- 

 tion," and accordingly these observations which he had been amassing upta 

 the reading of his first paper, he never has brought forward, although at the 

 time (it would appear from the words " at present ") he intended to have 

 done so. 



Secondly. He admits in his letters that I communicated experimental me- 

 thods to him, and there is not a single xuurd in those letters implying evpn 

 that he had contemplated anything of the sort, much less that he had spoken 

 to any one previously on the subject ; and surely if he had done so he 

 would naturally have mentioned \t when I suggested the methods in ques- 

 tion, as he has admitted in his fifth letter. On the contrary, when I gave 

 him the proportion D : D — d : : moisture of saturation : actual moisture, 

 in the N.B. to his first letter he denies its correctness, and states it to be 

 only true in a case in which (as I have shown) it would be wholly erroneous. 

 In his second letter he admits its correctness because he had deduced from his 

 formulathat D -.D — d : : f :/",a conclusionequicalenttomine; consequent- 

 ly, I presume, he had not deduced this when he wrote his first letter in which 

 he denied its correctness. In his fifth letter he said again that my propor- 

 tion was correct, /or it was easilij deducible from his formula ; but in Febru- 

 ary 1835 (see the extract in the preceding page) he says that such is not 

 the case, and attempts to account for his erroneous result. 



Thirdly. In his second letter hesays he can give the fall of the hygrometer 

 in dry air theoretically , and oflers to furnish me with such a table. Would 

 not this appear to iw/j/y that he was .sr(<(.s;^"e(/ with the correctness of his 

 </jeore<(ca/ tY//«cs, and had no ideaofinstitutingexperiments himself, rt/</ioj<g^, 

 as he says, " he would be happy if I would undertake it experimentally, as 

 even a few experimental results would be highly interesting." ? 



