MR. J. H. BRINKWORTH ON SPECIFIC HEAT OF STEAM. 395 



different ranges. The mean result found in this way was S = 2'074, corresponding to 

 S = 0'496 calories per 1 C., which agreed as closely as could be expected with my 

 previous experiments by other methods, giving S = 0'497 over the same range. A 

 systematic variation depending on the initial temperature was subsequently noticed, 

 which requires further explanation, and which formed the starting point of fresh 

 investigations. 



Effect of Impurities in the Steam. 



Steam in the immediate neighbourhood of the saturation pointl is liable to' carry 

 small particles of water in suspension, which cannot be evaporated completely by a 

 moderate degree of superheat if any impurities, such as salt in solution, are present. 

 Since 1 mgr. of water requires more than half a calorie to evaporate it, and the heat 

 required to raise the temperature of 1 gr. of steam 10 C. is only 5 calories, it is 

 necessary that the initial steam should not contain more than 1 in 100,000 of water, 

 if the specific heat is to be found correct to 1 in 1000 over a range of 10 C. 



The rise of boiling point 6 produced by .? gram-molecules of salt per gram of water 

 is approximately lOOO.r C. The proportion of suspended water remaining un- 

 evaporated at any degree of superheat 0' will be 1000x/0'. The quantity evaporated 

 in heating the steam from 6' to 6" will be lOOO.r (9"-9')/9'9". This will produce an 

 apparent increase of the mean specific heat of the steam over the range 9" 9' 

 equivalent to 1000L,r/0V, where L is the latent heat of evaporation. It was found 

 that this extremely simple and convenient reduction formula fitted the results 

 obtained over different ranges of temperature with extraordinary precision, and 

 reconciled apparent discrepancies which had previously been attributed to errors of 

 observation. A few examples of this reduction are given in the preceding table, e.g., 

 the observations with flow (2) for different ranges of temperature on July IGtli, which 

 are seen to agree extremely well when reduced to a common range 103 C. to 1 13 C. by 

 this formula. The numerical value of the constant lOOOLx in the reduction formula was 

 found to be 173 joules, which would be equivalent to the presence in the steam of an 

 impurity of about half a millionth of a gram-molecule of salt per gram. It is quite 

 possible that an impurity of this order of magnitude might have been produced by the 

 continued use of ordinary water in the boiler, and the passage of the steam through 

 considerable lengths of rubber and glass tubing. Small variations in the impurity 

 might also account for some of the differences in the results obtained on different 

 occasions, but as the same boiler and regulator were employed in all cases, the 

 variations in the quality of the steam supplied would probably be unimportant. 



The effect of this reduction is evidently greatest when the superheat is small. Any 

 small change in the initial superheat 9' was found to produce a relatively large effect 

 on the results. The length of the steam jacket cannot allow sufficient time for 

 raising the steam exactly to the temperature of the jacket before entering the 

 calorimeter. There is always a systematic fall of initial temperature with increase of. 



