358 



Lord Rayleigh. On the relative [Feb. 9, 



what the comparability of the weighings, vacuous and full, on which 

 everything depends. Hydrogen is so light that, except perhaps in 

 the mean of a long series, the error of weighing is likely to be more 

 serious than the uncertainty of temperature. I have therefore con- 

 tented myself with enclosing the body of the globe during the process 

 of filling in a wooden box, into which passed the bulbs of two ther- 

 mometers, reading to tenths of a degree centigrade. It seems probable 

 that the mean of the readings represents the temperature of the gas 

 to about yoth degree, or at any rate that the differences of temperature 

 on various occasions and with various gases will be given to at least 

 this degree of accuracy. Indeed the results obtained with oxygen 

 exclude a greater uncertainty. 



Under these conditions the alternate full and empty weighings can 

 be effected with the minimum of interference with the surface of the 

 globe. The stalk and tap were only touched with a glove, and the 

 body of the globe was scarcely touched at all. To make the symmetry 

 as complete as possible, the counterpoising globe was provided with 

 a similar case, and was carried backwards and forwards between the 

 balance room and the laboratory exactly as was necessary for the 

 working globe. 



In my earliest experiments (1885) hydrogen and oxygen were pre- 

 pared simultaneously in a [J -shaped voltameter containing dilute sul- 

 phuric acid. Since the same quantity of acid can be used indefinitely, 

 I hoped in this way to eliminate all extraneous impurity, and to obtain 

 hydrogen contaminated only by small quantities of oxygen, and vice 

 versa. The final purification of the gases was to be effected by passing 

 them through red-hot tubes, and subsequent desiccation with phos- 

 phoric anhydride. In a few trials I did not succeed in obtaining good 

 hydrogen, a result which I was inclined to attribute to the inadequacy 

 of a red heat to effect the combination of the small residue of oxygen.* 

 Meeting this difficulty, I abandoned the method for a time, purposing 

 to recur to it after I had obtained experience with the more usual 

 methods of preparing the gases. In this part of the investigation 

 my experience runs nearly parallel with that of Professor Cooke. The 

 difficulty of getting quit of the dissolved air when, as in the ordinary 

 preparation of hydrogen, the acid is fed in slowly at the time of 

 working, induced me to design an apparatus whose action can be 

 suspended by breaking an external electrical contact. It may be 

 regarded as a Smee cell thoroughly enclosed. Two points of difference 

 may be noted between this apparatus and that of Professor Cooke. In 

 my manner of working it was necessary that the generator should 



* From Professor Cooke's experience it appears not improbable that the impurity 

 may have been sulphurous acid. Is it certain that in his combustions no hydrogen 

 (towards the close largely diluted with nitrogen) escapes the action of the cupric 

 oxide ? 



