March i, 1888] 



NATURE 



419 



and the other to the floor. When the galvanometer in- 

 dicated that the ceihng was the warmer, the balance 

 behaved well, and vice versa. The reason is of course 

 that air is stable when the temperature increases upwards, 

 and unstable when heat is communicated below. During 

 the winter months the ground was usually warmer than 

 the rest of the room, and air currents developed them- 

 selves in the weighing closet. During the summer the 

 air cooled by contact with the ground remained as a 

 layer below, and the balance was undisturbed. 



The principal difference to be noted between my 

 arrangements and those of Prof. Cooke is that in my 

 case no desiccators were used within the weighing closet. 

 The general air of the room was prevented from getting 

 too damp by means of a large blanket, occasionally removed 

 and dried before a fire.^ 



In Regnault's experiments the globe was filled with gas 

 to the atmospheric pressure (determined by an inde- 

 pendent barometer), and the temperature was maintained 

 at zero by a bath of ice. The use of ice is no doubt to 

 be recommended in the case of the heavier gases ; but it 

 involves a cleaning of the globe, and therefore diminishes 

 somewhat 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 inclosing the body of the globe during 

 the process of filling in a wooden box, into which passed 

 the bulbs of two thermometers, reading to tenths of a 

 degree centigrade. It seems probable that the mean of 

 the readings represents the temperature of the gas to 

 about one-tenth of a degree, or at any rate that the differ- 

 ences 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 inter- 

 ference with the surface of the globe. The stalk and 

 tap were only touched with a glove, and the body of the 

 ^lobe was scarcely touched at all. To make the symmetry 

 as complete as possible, the counterpoising globe was 

 provided with a similar case, and was cairied backwards 

 and forwards between the balance room and the labora- 

 tory exactly as was necessary for the working g'obe. 



In my earliest experiments (1885) hydrogen and oxygen 

 were prepared simultaneously in a U-shaped voltameter 

 containing dilute sulphuric 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 

 -oersd. The final purification of the gases was to be 

 •effected by passing them through red-hot tubes, and sub- 

 sequent desiccation with phosphoric anhydride. In a l^w 

 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 difificulty, I abanioned the 

 method for a time, purposing to recur to it after I had 

 obtained experience with the more usual methods of pre- 

 paring the gases. In this part of the investigation my 

 experience runs nearly parallel with that of Prof. 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 inclosed. Two points of difference may 



' I can strongly recommend this method. In twenty-four hours the blanket 

 will frequently absorb twj pounds of moisture. 



^ From Prjf. Cooke'.s e.vperienc : it appears nit improbable that the 

 1 mpurity may have been sulphurous acid. Is it certain that in his combus- 

 tions no hydrogen (towa-ds the close largely diluted with nitrogen) escapes ' 

 the action of the cuprx oxide ? I 



be noted between this apparatus and that of Prof. Cooke- 

 In my mnnner of working it was necessary that the 

 generator should stand an internal vacuum. To guard 

 more thoroughly against the penetration of external air, 

 every cemented joint was completely covered with vase- 

 line, and the vaseline again with water. Again, the zincs 

 were in the form of solid sheets, closely surrounding the 

 platinized plate on which the hydrogen was liberated, 

 and standing in mercury. It was found far better to work 

 thcs2 cells by their own electromotive force, without 

 stimulation by an external battery. If the plates are close, 

 and the contact wires thick, the evolution of gas may be 

 made more rapid than is necessary, or indeed desirable. 



Tubes, closed by drowned stopcocks, are provided, in 

 order to allow the acid to be renewed without breaking 

 joints ; but one charge is sufficient for a set of experi- 

 ments (three to five fillings), and during the whole of the 

 time occupied (10 to 14 days) there is no access of 

 atmospheric air. The removal of dissolved air (and other 

 volatile impurity) proved, however, not to be so easy as 

 had been expected, even when assisted by repeated ex- 

 haustions, with intermittent evolution of hydrogen ; and 

 the results often showed a progressive improvement 

 in the hydrogen, even after a somewhat prolonged 

 preliminary treatment. In subsequent experimeats 

 greater precautions will be taken.^ Experience showed 

 that good hydrogen could not thus be obtained 

 from zinc and ordinary "pure" sulphuric acid, or 

 phosphoric acid without the aid of purifying agents. 

 The best results so far have been from sulphuric and 

 hydrochloric acid, when the gas is passed in succession 

 over liquid potash, through powdered corrosive sublimate, 

 and then through powdered caustic potash. All the joints 

 of the purifying tubes are connected by fusion, and a tap 

 separates the damp from the dry side of the apparatus. 

 The latter includes a large and long tube charged with 

 phosphoric anhydride, a cotton-wool filter, a blow-off 

 tube sealed with mercury until the filling is completed, 

 besides the globe itself and the Toppler pump. A detailed 

 description i.s postponed until the experiments are com- 

 plete. It may be sufficient to mention that there is but 

 one india-rubber connection — that between the globe and 

 the rest of the apparatus, and that the leakage through 

 this was usually measured by the Toppler before com- 

 mencing a filling or an evacuation. 



The object of giving a considerable capacity to the 

 phosphoric tube was to provide against the danger of a 

 too rapid passage of gas through the purifying tubes at 

 the commencement of a filling. Suppose the gas to be 

 blowing off, all the apparatus except the globe (and the 

 Toppler) being at a pressure somewhat above the atmo- 

 spheric. The tap between the damp and dry sides is 

 then closed, and that into the globe is opened. The gas 

 which now enters somewhat rapidly is thoroughly dry, 

 having been in good contact with the phosphoric an- 

 hydride. In this way the pressure on the dry side is 

 reduced to about 2 inches of mercury, but this residue is 

 sufficient to allow the damp side of the apparatus to be 

 exhausted to a still lower pressure before the tap between 

 the two sides of the apparatus is reopened. When this 

 is done, the first movement of the gas is retrograde ; and 

 there is no danger at any stage of .imperfect purification. 

 The generator is then re-started until the gas (after from 

 two to five hours) begins to blow off again. 



In closing the globe, some precaution is required to 

 secure that the pressure therein shall really be that mea- 

 sured by the barometer. The mercury seal is at some 

 distance from, and at a lower level than, the rest of the 

 apparatus. After removal of the mercury, the flow of 

 gas is continued for about one minute, and then the tap 

 between the dry and damp sides is closed. From three 

 to five minutes more were usually allowed for the com- 



' Spectrum analysis appears t ) be incapable of indicating the presence of 

 comparatively large quantities of nitrogen. 



