fusion, except that the tube of infusible glass containing the copper oxide was joined to the soft glass of the rest of the apparatus by means of ground joints made gas-tight with paraffin. The pure dry oxygen issuing from the last drying tube passed to a manometer indicating its pressure, and to a stopcock separating the apparatus up to this point from that on the other side of the stopcock, where the pressure was less until the completion of the process of filling the globe with oxygen. Beyond the stopcock, a fusible metal plug closed the tube leading to the globe until the exhaustion was finished. The tube between this stopcock and the fusible metal plug was freed from air by exhaustion with another pump, after which the tube leading to this pump was closed by fusion. When the required exhaustion had been obtained, this plug was removed, and the passage of the oxygen to the globe was regulated by a stopcock. The manometer which showed the pressure of the of the gas in the voltameter was connected by so long a tube, and oxygen was so often blown out at it, that it was thought impossible for mercurial vapor to mingle with the current of the gas on its way to the globe.

36.-OXYGEN BY THIRD METHOD.

MEASUREMENT OF PRESSURE.

The measurement of pressure in this series was like that in the first series, except in one particular. In the first series I used what Regnault called a manobarometer, the relation of the tubes in which is well known. But in the present case I used a syphon barometer, as shown diagrammatically in Fig. 22. In a box stand two tubes, e and f, one connected to the globe, and the other filled with mercury by boiling in a vacuum. The upper part of e and the tube fare in the same perpendicular. The difference of level which measures the pressure of the oxygen in the globe can be accurately and conveniently measured by means of a scale standing in contact with the two tubes and viewed with proper optical apparatus.

37.-OXYGEN BY THIRD METHOD.

WEIGHING THE GLOBES.

The globes, after washing and wiping, were hung with their counterpoises in desiccators in the closet under the balance. But the desiccators now used were not required to hold the globes permanently, and were therefore simple boxes of sheet metal, with a cover having holes for the suspension of the globes from the balance. The globes were placed in one of these desiccators, the cover was put on, the box was put in position on the rotating platform of the reversal mechanism of the balance, and the wires attached to the globes were hung to the auxiliary pans of the reversal apparatus. Dry air was then admitted at the bottom of the desiccator and there spread horizontally in all directions. This current was introduced

FIG. 22a.-Desiccator.

through the axis of the reversal apparatus, and could therefore be continued without interruption except for the purpose of weighing.

The volumes suspended from the opposite pans of the balance were always made equal, as in the previous series of determinations, and the weighings were made by reversal in precisely the same way.

38.-OXYGEN BY THIRD METHOD. WEIGHT OF GLOBES WHEN EXHAUSTED.

As already observed, it was thought proper to determine the amount of oxygen in the globe by determining the difference of weight between the filled globe and the globe exhausted immediately afterwards. In this exhaustion and the subsequent weighing, leakage through the stopcock of the globe was prevented by the manipulation described on page 47. Now, since the tare of the globe thus obtained was only for the one experiment immediately preceding, the weight of the joint shown in Fig. 20, and used to prevent leakage, did not need to be known. The joint was therefore made ready for application to the globe when the latter was to be weighed with its contents of oxygen, and was, during that weighing, put on the balance with the globe. After the weighing was completed, the joint was put on the globe, a vacuum was produced, the joint was closed by fusion, and the globe with the joint was weighed again. The difference of these two weighings (in which the volumes on the opposite pans were made equal as usual) gave the weight of the oxygen contained in the globe.

Since there was no room for any exercise of judgment in the combination of observations made in this way, it is sufficient to give simply the number of the globe employed, the pressure, the weight of oxygen, and the density of oxygen found. The capacities of the globes have already been given.

39.-OXYGEN BY THIRD METHOD.

OBSERVATIONS AND RESULTS.

The results obtained by the method in which the globes were filled with oxygen at the temperature of melting ice were as follows. It may be said that three observations were made with the globe numbered 6. But it was found that an error had been made in the computation of the volume of its counterpoise, so that all the observations made with it were subject to an uncertain and variable correction for the difference of the volume of the globe and its counterpoises. These three results were therefore rejected.

If we increase the mean by one thirty-thousandth,* we get

D= 1.42917 gr. 0.000048.

40.-OXYGEN. FINAL RESULT FOR THE DENSITY.

The values found by the different methods used are as follows:

By use of thermometer and manometer..
By compensation.....

By use of ice and barometer.

D = 1.42879.00003 4

D =

1.42887 ±.00004 8
D = I. .42917
±.00004 8

The combination of these results into a final mean must be left mostly to the judg ment of those interested in the matter. The probable errors of the three means would indicate that the first method should have double weight, which would be very improper, in my judgment. This method involves more easy manipulation, but gives less security against constant error. So also with the second method. The manipulation of the third involves more accidental errors, but involves no constant errors which are not common to the other methods, while it avoids some. I shall therefore compute a mean in which the third result is given double weight; from which we get

55

PART II.--ON THE DENSITY OF HYDROGEN.

1.--INTRODUCTION.

The density of hydrogen has been determined in five series of experiments of very unequal value. In the first series, temperature and pressure were measured with mercurial thermometers and the mano-barometer. The reduction took account. of the same factors as the reduction of the first series in the case of series consisted of fifteen separate determinations.

oxygen. This

In a second series, the globes were surrounded with melting ice while the pressure was measured with the syphon barometer. The reduction was the same as in the third series of experiments on oxygen. This series consisted of nineteen experiments.

In a third series the hydrogen was weighed, not in the globes where its volume, temperature, and pressure were observed, but before it was introduced into them. Globes making jointly a capacity of forty-two litres were connected together, and to a self-acting air pump, to a syphon barometer, and to a tube for admitting hydrogen. A tube containing six hundred grammes of palladium foil was charged with hydrogen, and was weighed. Its hydrogen was transferred to the globes, previously exhausted and shut off from the air-pump. Here its volume and pressure were determined at the temperature of melting ice. The tube of palladium was again weighed, and so the weight of hydrogen was determined. This series consisted of eight experiments, together with five experiments which were made before the apparatus and its manipulation were satisfactory, and which were regarded as only preliminary. The reduction of these observations takes account of the elevation of the cistern of the barometer above the centre of the globes and of the force of gravity at my laboratory.

The fourth series was simply a continuation of the third after a summer vacation. Several accidents occurred. The series was therefore abandoned, and a new apparatus was constructed. The fifth series was a repetition of the third and fourth with a different apparatus; these two series consisted of six and eleven experiments respectively, but accidents occurred in two experiments of the fourth series.

2.-DETERMINATION OF DENSITY BY FIRST METHOD.

In the first series of determinations of the density of hydrogen, nearly all the manipulation was almost precisely the same as that used in the first series of determinations of the density of oxygen. The two were carried on at the same time, with the same apparatus in the same condition; it was hoped that in this manner of working the ratio of the two densities might be subject to fewer systematic errors. Only a brief account is therefore required.

In all the experiments of this series, hydrogen was prepared by the electrolysis of pure dilute sulphuric acid. The gas was passed through a strong solution of potassium hydroxide, over incandescent copper, and through three drying tubes one metre long and two and a half centimetres in diameter, the last filled with phosphorus pentoxide and the preceding one with powdered potassium hydroxide. In the apparatus up to this point, the pressure was always kept equal to that of the atmosphere by a stopcock and a manometer.

In some experiments of this series the gas was admitted at once to the globe as it issued from the regulating stopcock. In the remaining experiments, the gas was first absorbed in metallic palladium. When this was to be filled with hydrogen, it was first heated in a vacuum. Then hydrogen was passed through it while it was still too hot to absorb the gas, and the current was continued for some time, after which the source of heat was removed. After the absorption was complete, a current of gas was passed for a long time to expel any gas other than hydrogen which might have accompanied the latter. When this end was thought to have been attained, the connection between the palladium and the voltameter was cut off, and the connection opened to deliver the hydrogen into the globe. Heat being applied to the palladium, the globe was filled.

In all the experiments in which palladium was used, the tubes for conducting the gas were closed, either by fusion or by fusible metal plugs, so that leakage from the atmosphere into the apparatus was absolutely excluded.

FIRST METHOD. PROOF OF THE PURITY OF THE HYDROGEN.

The voltameter in which hydrogen was produced contained about five litres of dilute sulphuric acid, and the drying tubes and other parts of the apparatus had a capacity of one or two litres more. To remove the air from this apparatus took of course a considerable time. It was thought proper to meet any objections