When the gases are mingled, it is obvious that the constants B and b will each enter into both equations; whether the same is true of A, and how, are perhaps uncertain. First supposing that it should not; when

The sum of these gives the pressure of the mixture, supposing that the oxygen and hydrogen were originally measured at unit pressure and then mixed without change of their joint volume; therefore

2
P+p=1+ (A + a).

9

This is on the supposition that no mutual attraction acts between the unlike molecules. If such attractions exist, and if they follow the law f =

take notice of them by writing

Mm

d2

we can

The value of A seems to depend on the temperature, so that it must be determined from observations of pressure and volume at 0°. The experiments of Amagat * serve to determine A and a. Sarrau + has discussed the first series, but with some modification of Van der Waals' formula. If we reduce the constants

* Annales de Chimie et de Physique (5), 22, 353.

Comptes Rendus, 94, 844.

computed by Sarrau to those which would obtain in the unmodified equation, we have

A 0.0020
a = 0.0002

But, as Sarrau remarks, the proof that a has a sensible magnitude is insufficient. From Amagat's later series of observations,* no value of A can be computed which will satisfy the determinations at all pressures, as Professor William Harkness has assured me. That Nan der Waals' equation needs modification is attested by the more or less successful attempts of Clausius, Sarrau, and Amagat. But it will answer our purpose, provided that we compute its constants from observations at pressures not far from those to which we desire to apply it. If we compute A and a from various combinations of observations at three different pressures, we get the following values:

VALUES OF A AND а FROM AMAGAT'S EXPERIMENTS,

Only the positive, or the numerically smaller negative, root is given. It is plain that constant values of A and a will not satisfy the observations; we can scarcely do better than to assume, with Sarrau,

Substituting these values in the expression for the pressure of the mixture of two volumes of hydrogen and one volume of oxygen when occupying three volumes we have

Pp 1.000389

We may therefore correct for the deviation of the mixed gases from the density computed by Boyle's law, by multiplying the observed density by the factor 1.000389; we thus find

If we use the values

()) = 1.42900 gr.,

H = .0898731gr.,
M = .535510 gr.,

we find the ratio of the volumes of hydrogen and oxygen in the mixture whose density was determined :

Ratio of Mixture
2.00357.

Subtracting from this the excess of hydrogen found by analysis, we get the ratio of the combining volumes:

Ratio of Combining Volumes.

2.00269.

PART IV.--SYNTHESIS OF WATER FROM WEIGHED QUANTITIES

OF HYDROGEN AND OXYGEN.

1. INTRODUCTION.

In the following pages are described some experiments in which a quantity of hydrogen was weighed while absorbed in palladium, a quantity of oxygen was weighed in a globe, the two were combined, and the water produced was weighed.

The two gases were brought together at two platinum jets enclosed in a small glass apparatus, which was weighed while exhausted, where they were made to combine. When the combustion was ended, the gas remaining in this combustion apparatus and the connecting tubes was extracted with a Toepler air pump, measured, and analyzed. The combustion tube, the globe which had contained oxygen, and the palladium tube were weighed again. From the amounts of oxygen and hydrogen extracted were subtracted the amounts of the gases found in the analysis; the remainders were the quantities combined in the combustion apparatus, from which the atomic weight of oxygen was found. The amount of water produced was measured by the gain in weight of the combustion apparatus; dividing this by the amount of hydrogen used, the molecular weight of water was known, and so a second value for the atomic weight of oxygen was obtained.

The volume of hydrogen used in most of the experiments was forty-two or forty-three litres; the amount of water produced was about thirty-four grammes in each experiment; twelve successful experiments were made. The amount of gas left unburned and therefore measured in the eudiometer varied from a six-hundredth to a ten-thousandth of the quantity concerned.

2.--PRODUCTION AND WEIGHING OF OXYGEN.

The oxygen used in all the experiments of this series was produced by heating potassium chlorate; the apparatus used is shown in Fig. 7, page 20; the manipu lation here was precisely the same as in the other experiments. In fact, often the same quantity of oxygen served for a determination of density and for a synthesis

of water.

After the tube containing the chlorate and the whole apparatus and its connecting tubes had been exhausted, the chlorate was heated till oxygen came off freely, when it was cooled, and the apparatus again exhausted. The gas passed through three tubes each a metre long and two and a half centimetres in diameter; the first was filled with beads moistened with a strong solution of potassium hy droxide, the second with beads and sulphuric acid, the third with phosphorus pentoxide and glass wool so that no channel could form above the oxide. A regulating stopcock kept the pressure in the first part of the apparatus at about that of the atmosphere.

It was convenient to use in the combustion the oxygen contained in two globes; they were commonly filled at the same time.

3. SYNTHESIS OF WATER. BALANCE AND WEIGHING ON REVERSAL APPARATUS.

All the weighings of this series were made on the Becker balance carrying twelve hundred grammes, which was mentioned at page 28, and shown in Fig. 10, page 30. It was mounted on the closet shown in Fig. 16, page 38. It was provided with the apparatus for weighing by reversal which is shown in Figs. 16 to 19. But at that time, some trifling details were not quite as shown by the, drawings; the operation of weighing was, however, effected exactly as by the apparatus in its present condition.

4.--SYNTHESIS OF WATER.

MANIPULATION OF TUBE CONTAINING PALLADIUM.

These experiments were the first in which large quantities of hydrogen were weighed while absorbed in palladium; so some changes were by degrees introduced into the apparatus. As far as I see, these trifling modifications simply made the manipulation shorter and more convenient, without affecting the accuracy of the results. But opinions may differ on this matter, so that it is proper to describe the three forms which the tube containing palladium successively assumed.

a

a

In experiments 1 and 2, the palladium was contained in two separate hard glass tubes, Fig. 33. To each was fitted a stopcock of common glass by means of a ground joint and paraffin. A stopcock of hard glass cannot be made. The manipulation of this tube would have been very simple if a stopcock were anything more than a contrivance for lessening the flow of a gas through a tube. with freedom from large constant error, in manipulation with a gas so light as hydrogen, it was necessary that leakage should be rendered excessively improba

FIG. 33.-Palladium tube, first form.

But for any precision, together