844; Prof. Apjohn on the Specific Heats of the Gases, 



cumstance has been overlooked by M. Suerman, and he has 

 as a consequence deduced from my experiments numbers for 

 the specific heats of the different gases materially higher than 

 the true. This is the chief cause of the difference between 

 his numbers and mine as they appear in his thesis 6*89 ; but 

 the difference in question is also partly attributable to the 

 circumstance of his having had access only to my first series 

 of experiments. 



The numbers then which I have obtained are 8*6, the same 

 with Suerman's. This is true of them relatively but not abso- 

 lutely, for his values of a for air and the different gases are 

 in every instance greater than mine. This is partly owing, 

 as has been already explained, to the use of the division 'p—J' 

 instead of p in his formula, and to his assigning to e a higher 

 value than I have given it. The chief cause, however, is that 

 in his experiments the depressions were in no instance as great 

 as those which I obtained; and this brings me to observe 

 upon what I consider as a great defect in the apparatus which 

 he employed. The tube containing the thermometers was, 

 I conceive, too large, and its shape badly chosen, so that from 

 both circumstances combined, it was scarcely possible that the 

 air brought once in contact with the moistened bulb should 

 be with certainty immediately displaced. The current in fact, 

 which swept by the thermometers, being intermixed with gas 

 already in the tube in a state of comparative stagnation, and 

 which gas had necessarily acquired moisture from the wet ther- 

 mometer, the depression indicated by the latter instrument 

 was not that due to the elastic fluid in a state of perfect desic- 

 cation, but the feebler one caused by gas already charged with 

 a certain amount of humidity. It is very true, as Mr. Suerman 

 remarks, p. 23, that the diameter of the tube containing the 

 thermometers cannot be reduced beyond a certain point, for in 

 such case, unless indeed the rapidity of the current be greatly 

 augmented, the effect of the radiation of the sides of the tube 

 upon the thermometer would become very sensible, and cause 

 the instrument prematurely to reach its stationary condition. 

 My objection, however, is not so much to the absolute as to 

 the relative magnitude of the tube in question. It was larger 

 than that of the tubes which convey the gas into and from it, 

 and hence the permanent contact of a certain quantity of 

 moist air with the bulb of the wet thermometer was inevitable. 

 In my experiments the two thermometers were placed length- 

 wise in a glass tube whose calibre was somewhat less than that 

 of the other passages of the apparatus, so that the column of 

 gas which it included was necessarily altogether successively 



