226 



Mr. J. Joly. 



For air at 



■j 



0° the relative density is 0*00069, 

 5° „ „ 0*00066, 



10° „ „ 0-00064, 



15° „ „ 0-00062, 



20° „ „ 0-00060. 



The volume of the substance in cubic centimetres is to be multi- 

 plied by the most suitable of these factors to ascertain the amount to 

 be deducted from the apparent weight of condensation. 



Correction for the Carrier. — A deduction, from the observed weight 

 of precipitation, due to the calorific capacity of the carrier is of 

 course necessary. This is effected on a previous experiment (or ex- 

 periments) on the empty carrier and proportionately to the relative 

 extent of the ranges in the two cases. 



On the Accuracy of the Method and the Error arising from Radiation. 



Many experiments bearing on the accuracy of the method are con- 

 tained in my former papers. It is sufficient to say here that: — (1.) 

 Successive experiments on the same piece of matter, whether a good 

 conductor of heat or a bad conductor, show one with another a con- 

 sistency of result exceeding that found in the records of observations 

 by other methods, as in Regnault's experiments, using the method 

 of mixtures on a very elaborate scale, and dealing with very large 

 quantities of matter. (2.) The results obtained, both with good 

 conductors and bad conductors, agree closely with the most reliable 

 determinations of Regnault, Bede, Mallet, &c. (3.) Wide variations 

 in extent of surface, and in the quantity of the substance placed in 

 the calorimeter, fail to affect the consistency of the result. I will 

 explain with one example. A limpid crystal of barytes weighing 

 76*109 grams, placed in the calorimeter, afforded 0'10923 as the mean 

 specific heat between 9*65° and 100*30°. It was now broken up into 

 small fragments, which were piled up on the carrier : 65*143 grams 

 were thus returned to the calorimeter. These afforded 0*10910 as 

 the mean specific heat over the range 9*60° to 99'80°. 



This no mere accidental coincidence.* It is certainly exceptional 

 for repetition experiments to differ by more than half per cent. With 

 ordinary care, indeed, they are quite as faithful and as sure as 

 repetition determinations of specific gravity made in the ordinary 

 way, and on similar quantities of matter. 



It is to be added that since the experimental evidence in support 

 of the method was published much has been done, using various 

 modifications of the apparatus. I have found that the results in 



* A table containing many such experiments is contained in my paper " On the 

 Method of Condensation," p. 362. 



