it seemed possible that each group was not contributing its proper share, and even that there might be a flow in the wrong direction at the delivery end of one or two of them. To meet this objection, the arrangement in parallel had to be abandoned, and for the remaining experiments 8 pipes were connected in simple series. The porous surface in operation was thus reduced, but this was partly compensated for by an improved vacuum. Two experiments were made under the new conditions:

The excess being larger than before is doubtless due to the greater efficiency of the atmolysing apparatus. It should be mentioned that the above recorded experiments include all that have been tried, and the conclusion seems inevitable that "atmospheric nitrogen" is a mixture, and not a simple body.

It was hoped that the concentration of the heavier constituent would be sufficient to facilitate its preparation in a pure state by the use of prepared air in substitution for ordinary air in the oxygen apparatus. The advance of 33 mg. on the 11 mg. by which atmospheric nitrogen is heavier than chemical nitrogen, is indeed not to be despised, and the use of prepared air would be convenient if the diffusion apparatus could be set up on a large scale and be made thoroughly self-acting.

7. NEGATIVE EXPERIMENTS TO PROVE THAT ARGON IS NOT DERIVED FROM NITROGEN FROM CHEMICAL SOURCES.

Although the evidence of the existence of argon in the atmosphere, derived from the comparison of densities of atmospheric and chemical nitrogen and from the diffusion experiments, appeared overwhelming, we have thought it undesirable to shrink from any labor that would tend to complete the verification. With this object in view, an experiment was undertaken and carried to a conclusion on November 13th, in which 3 litres of chemical nitrogen, prepared from ammonium nitrite, were treated with oxygen in precisely the manner in which atmospheric

nitrogen had been found to yield a residue of argon. In the course of operations an accident occurred, by which no gas could have been lost, but of such a nature that from 100 to 200 c. c. of air must have entered the working vessel. The gas remaining at the close of the large scale operations was worked up as usual with battery and coil until the spectrum showed only slight traces of the nitrogen lines. When cold, the residue measured 4 c. c. This was transferred, and after treatment with alkaline pyrogallate to remove oxygen, measured 3.5 c. c. If atmospheric nitrogen had been employed, the final residue should have been about 30 c. c. Of the 3.5 c. c. actually left, a good part is accounted for by the accident alluded to, and the result of the experiment is to show that argon is not formed by sparking a mixture of oxygen and chemical nitrogen.

A similar set of experiments was carried out with magnesium. The nitrogen, of which three litres were used, was prepared by the action of bleaching powder on ammonium chloride. It was circulated in the usual apparatus over red-hot magnesium, until its volume had been reduced to about 100 cubic centimetres. An equal volume of hydrogen was then added, owing to the impossibility of circulating a vacuum. The circulation then proceeded until all absorption had apparently stopped. The remaining gas was then passed over red-hot copper oxide into the Sprengel's pump, and collected. As it appeared still to contain hydrogen, which had escaped oxidation, owing to its great rarefaction, it was passed over copper oxide for a second and a third time. As there was still a residue, measuring 12.5 cubic centimetres, the gas was left in contact with red-hot magnesium for several hours, and then pumped out; its volume was then 4.5 cubic centimetres. Absorption was, however, still proceeding when the experiment terminated, for at a low pressure the rate is exceedingly slow. This gas, on being examined with the spectroscope, contained both hydrogen and nitrogen, and failed to show the red lines of argon. The amount of residue attainable from three litres of atmospheric nitrogen should have amounted to a large multiple of the quantity actually obtained.

8. SEPARATION OF ARGON ON A LARGE SCALE.

To separate nitrogen from "atmospheric" nitrogen on a large scale, by help of magnesium, several devices were tried. It is not necessary to describe them all in detail. Suffice it to say that an attempt was made to cause a store of "atmospheric nitrogen" to circulate by means of a fan, driven by a water-motor. The difficulty encountered here was leakage at the bearing of the fan, and the introduced air, on coming into contact with the magnesium, produced a cake which blocked the tube. It might have been possible to remove oxygen by metallic

copper; but instead of thus complicating the apparatus, a water-injector was made use of to induce circulation. Here also it is unnecessary to enter into details. For, though the plan worked well, and although about 120 litres of "atmospheric " nitrogen were absorbed, the yield of argon was not large, about 600 cubic centimetres having been collected. This loss was subsequently discovered to be due partially at least to the relatively high solubility of argon in water. In order to propel the gas over magnesium, through a long combustion-tube packed with turnings, a considerable water-pressure, involving a large flow of water, was necessary. The gas was brought into intimate contact with this water, and presuming that several thousand litres of water ran through the injector, it is obvious that a not inconsiderable amount of argon must have dissolved. Its proportion was increasing at each circulation, and consequently its partial pressure also increased. Hence, towards the end of the operation, at least, there is every reason to believe that a serious loss had occurred.

It was next attempted to pass "atmospheric nitrogen" from a gas-holder first through a combustion-tube of the usual length packed with metallic copper reduced from the oxide; then through a small U-tube containing a little water, which was intended as an index of the rate of flow; the gas was then dried by passage through tubes filled with soda-lime and phosphoric anhydride, and it next passed through a long iron tube (gas-pipe) packed with magnesium turnings, and heated to bright redness in a second combustion-furnace. After the iron tube followed a second small U-tube containing water, intended to indicate the rate at which the argon escaped into a small gas-holder placed to receive it. The nitrogen. was absorbed rapidly, and argon entered the small gas-holder. But there was reason to suspect that the iron tube is permeable by argon at a red heat. The first tubefull allowed very little argon to pass. After it had been removed and replaced by a second, the same thing was noticed. The first tube was difficult to clean; the nitride of magnesium forms a cake on the interior of the tube, and it was very difficult to remove it; moreover, this rendered the filling of the tube troublesome, inasmuch as its interior was so rough that the magnesium turnings could only with difficulty be forced down. However, the permeability to argon, if such be the case, appeared to have decreased. The iron tube was coated internally with a skin of magnesium nitride, which appeared to diminish its permeability to argon. After all the magnesium in the tube had been converted into nitride (and this was easily known, because a bright glow proceeded gradually from one end of the tube to the other), the argon remaining in the iron tube was "washed" out by a current of nitrogen; so that, after a number of operations, the small gas-holder contained a mixture of argon with a considerable quantity of nitrogen.

On the whole, the use of iron tubes is not to be recommended, owing to the difficulty in cleaning them, and the possible loss through their permeability to argon. There is no such risk of loss with glass tubes, but each operation requires a new tube, and the cost of the glass is considerable if much nitrogen is to be absorbed. Tubes of porcelain were not tried; but they are hardly likely to suc ceed, for the glaze in the interior would certainly be destroyed by the action of the red-hot magnesium, and the roughening of the surface would render them as difficult to empty and refill as the iron tubes were found to be.

By these processes 157 litres of "atmospheric nitrogen" were reduced in volume to about 2.5 litres in all of a mixture of nitrogen and argon. This mixture was afterwards circulated over red-hot magnesium in order to remove the last portions of nitrogen,

As the apparatus employed for this purpose proved very convenient, a full description of its construction is here given. A diagram is shown in Fig. 4

FIG. 4.

which sufficiently explains the arrangement of the apparatus. A is the circulator. It consists of a sort of Sprengel's pump (a), to which a supply of mercury is admitted from a small reservoir (). This mercury is delivered into a gas-separator (c), and the mercury overflows into the reservoir (d). When its level rises, so that it blocks the tube (f), it ascends in pellets or pistons into a reservoir (e) which is connected through (g) with a water-pump. The mercury falls into (b), and again passes down the Sprengel-tube (a). No attention is therefore required, for the

apparatus works quite automatically. This form of apparatus was first suggested by Dr. Collie.

The gas is drawn from the gas-holder B, and passes through a tube C, which is heated to redness by a long-flame burner, and which contains in one half metallic copper, and in the other half copper oxide. This precaution is taken in order to remove any oxygen which may possibly be present, and also any hydrogen or hydrocarbon. In practice, it was never found that the copper became oxidized, or the oxide reduced. It is, however, useful to guard against any possible contamination. The gas next traversed a drying-tube D, the anterior portion containing ignited soda-lime, and the posterior portion phosphoric anhydride. It then passed through E, a piece of combustion-tube, drawn out at both ends, filled with magnesium turnings, and heated by a long-flame burner to redness. After regis tering the rate of circulation by passing a small U-tube with bulbs, filled with water, it again entered the gas-holder B.

After the magnesium-tube E had done its work, the stopcocks were all closed and the gas was turned down, so that the burners might cool. The mixture of argon and nitrogen remaining in the system of tubes was pumped out by a Sprengel's pump through F, collected in a large test tube, and reintroduced into the gas-holder B through the side-tube G, which requires no description. The magnesium-tube was then replaced by a fresh one; the system of tubes was exhausted of air; argon and nitrogen were admitted from the gas-holder B; the copper oxide-tube and the magnesium-tube were again heated; and the operation was repeated until absorption ceased. It was easy to decide when this point had been reached, by making use of the graduated cylinder H, from which water entered the gas-holder B. It was found advisable to keep all the water employed in these operations, for it had become saturated with argon. If gas was withdrawn from the gas-holder, its place was taken by this saturated water.

The absorption of nitrogen proceeds very slowly towards the end of the oper ation, and the diminution in volume of the gas is not greater than four or five cubic centimetres per hour. hour. It is therefore somewhat difficult to judge of the end-point, as will be seen when experiments on the density of this gas are described. The magnesium-tube, towards the end of the operations, was made so hot that the metal was melted in the lower part of the tube, and sublimed in the upper part. The argon and residual nitrogen had therefore been thoroughly mixed with gaseous magnesium, during its passage through the tube E.

To avoid possible contamination with air in the Sprengel's pump, the last portion of gas collected from the system of tubes was not readmitted to the gas-holder B, but was separately stored.