be made weaker, and then concentrated, but the adjustment is troublesome, and the formation of hard cakes of sodium sulphate, which interferes with the proper working at the next stage of the operation, is difficult to avoid.

The contents of the flask are now well drained into a basin, preferably a hemispherical nickel basin, or, lacking that, a stout porcelain one, capable of holding about 500 c.c. Potassium hydroxide, free from chloride, assayed for real alkali and for water, and having not less than two-thirds and not more than 1 mol. of water to 1 of the hydroxide, is required, for if it were anhydrous it would cause much heating and consequent decomposition of the salts; generally, the potassium hydroxide purified by alcohol and the more translucent varieties of stick potash contain about the right proportion of water, and dissolve in water without much rise of temperature. From 130 to 165 grams of it, according to its degree of hydration, are quickly crushed in a warm mortar, thrown into the solution in the basin, and incorporated with it by means of a pestle. There is marked heating only just at first, which is better met by keeping the basin in water or resting it on snow or pounded ice for a very short time. On stirring in the potassium hydroxide, the solution sets to a stiff paste, if kept cold, quickly becoming thin again on further stirring, but full of an opaque, white precipitate of sulphate. If the basin has been cooled, hardly any gas escapes at first, but gentle effervescence and much frothing occur before long in any case. When the potassium hydroxide has been all ground up and dissolved, the basin is placed under close cover from atmospheric moisture and carbonic acid, and left in a warm place for 30 hours; if kept for more than 50 hours, the quantity of hyponitrite sensibly, but slowly, diminishes. As much even as one-fourth of the hydroxyamidosulphonate may sometimes, in cold weather, still be present, and can be partly destroyed by keeping the basin at 55-60° for half an hour, although not with noticeable increase of the quantity of hyponitrite, but this heating, with the attendant risk of over-heating, is better omitted, on the whole. Besides undecomposed hydroxyamidosulphonate, the contents of the basin now consist of precipitated sulphate and sulphite, and solution of potassium hydroxide in slightly less than its weight of water (almost exactly KOH: 3H,O), together with the potassium hyponitrite. It is, apparently, only to secure this concentration of the potassium hydroxide, a practically saturated solution, that hardly less than 10 mols. of it to 1 of hydroxyamidosulphonate have to be used. More of it may be added without effect, good or bad, unless the solution of the salts is weaker than is here recommended, for in that case additional potassium hydroxide must be used to bring the concentration to the right point.

Treatment with a silver salt is the only way of separating the hyponitrite from the other salts, and for this purpose the presence of the alkali is essential, together with large dilution when precipitating. The best way is to use the silver solution exceedingly dilute, because this checks the precipitation of silver oxide and sulphite until some time after all the hyponitrite has separated. Now the necessity for large dilution, and the advantage of still larger dilution, remove the only objection that can be raised to the use of silver sulphate instead of silver nitrate, and since it is generally important to feel assured that no trace of nitrate or nitrite can have been carried down with the hyponitrite, the sulphate should have the preference, although the nitrate can almost certainly be used with as good results. A cold saturated solution contains only 5 or 6 grams of the sulphate to the litre, and is most easily prepared by boiling excess of the salt with water and pouring the solution into an equal volume of cold water.

Whichever salt is used, the contents of the basin are first washed into a very capacious precipitating vessel, and the highly dilute silver solution is poured in until it ceases to produce any more black precipitate. When this is at all abundant, as it sometimes is in cold weather, an hour's interval is given for subsidence of most of it, the still dark solution is decanted, and the precipitate washed by decantation before rejecting it. With or without this interruption, the addition of the silver solution is continued until the bright yellow hyponitrite suddenly appears, and so long after as the joint precipitation of brown oxide can be easily checked by stirring. When the point is reached where the oxide only redissolves slowly and no longer gives place to a yellow one of hyponitrite, no more silver solution should be added. If much more were added, there would be permanent precipitation of silver oxide, which is apt to be accompanied by silver sulphite. The quantity of silver sulphate required may be as much us 40 grams, which means 7 or 8 litres of solution; if silver nitrate be used, about 44 grams will be wanted, dissolved in 4 litres, or more, of water.

Half-an-hour after precipitation, the solution is to be poured off, even though still a little turbid, and the precipitate washed by decantation, for there is a very slow deposition of a mirror of metallic silver from the sulphite solution, which goes on for days. In order to separate the hyponitrite from the metallic silver and its oxide, and perhaps chloride, it has to be dissolved in dilute acid and reprecipitated. If every trace of nitrite is to be kept out of the hyponitrite, nitric acid can hardly be used, because I find that it always contains some nitrous acid, and it is, therefore, necessary to use sulphuric acid. Since the hyponitrite must be kept in solution as short a time as possible, it is advisable to have the acid not very VOL. LXXV.

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dilute, in order to reduce the volume of liquid to be filtered. But high dilution is better, because the stability of hyponitrite falls off rapidly with increasing concentration; moreover, if the sulphuric acid is not dilute enough, silver sulphate will separate; a 1 per cent. solution of the acid, well cooled in ice, is suitable, some 5 litres of it being required. The precipitate should be treated with the acid in portions at a time, not all together; and, as far as possible, the undissolved precipitate should be kept off the filter until the last. For so long a filtration a Lunge filter-tube-extension of the funnel is more suitable than the filter pump, the filtrate being allowed to fall directly into excess of sodium carbonate solution. Working with these precautions, the silver hyponitrite can be dissolved and reprecipitated, even in hot weather, with hardly any appreciable loss.

Having replaced the alkaline mother liquor by water, dilute sulphuric acid is cautiously added, until, after stirring up the precipitate well, the solution is no longer alkaline, and some of it, when filtered, is found to contain a trace of the silver hyponitrite; this is best ascertained by adding one or two drops of sodium carbonate solution to about 100 c.c. of it, which should cause a permanent, yellow, very slight, opalescence.

The precipitate, thoroughly washed by decantation and dried on a filter at the ordinary temperature in a desiccator, in the dark, and then at 100°, will give 78 per cent. silver (calc. 78 26). But in order to preserve the bright colour of the salt and its entire freedom from nitrite, all work on it should be done with very little exposure to bright daylight. The weight of silver hyponitrite obtained from the quantity of sodium nitrite employed should be not less than 17 grams.

IMPERIAL TOKYO UNIVERSITY, JAPAN.

XI.-Absorption of Nitric Oxide in Gas Analysis. By EDWARD DIVERS.

It is well known that the vapour tension of nitric oxide, dissolved in the solution of a ferrous salt, interferes with the use of this reagent to remove nitric oxide from other gases. There is, however, another absorbent for nitric oxide which leaves nothing to be desired, whose use and value have remained unknown. This is a strong solution of either sodium or potassium sulphite to which a little alkali hydroxide has been added. It quickly absorbs every trace of nitric oxide, which it fixes in the form of hyponitrososulphate, Na,N2O2SO. I have

already made satisfactory use of it to analyse the mixture of nitric oxide and nitrogen which is left on heating silver hyponitrite and allowing the solid and gaseous products to cool in contact with each other. The sulphite need not be very pure, the presence of sulphate or carbonate being of no importance. If carbon dioxide or other acid gas is present along with the nitric oxide, it is removed by alkali before using the sulphite mixture.

XII.-Interaction of Nitric Oxide with Silver Nitrate. By EDWARD DIVERS.

HAVING reason to think that silver nitrate might interact with nitric oxide if heated in it, and there being no information obtainable on this point, I have made some experiments on the action of nitric oxide on silver nitrate, as well as on other nitrates.

In the first place, something had to be ascertained as to the behaviour of silver nitrate when heated alone. Heated for 15 minutes in dry air or carbon dioxide, it suffers no chemical change until the temperature is close to the melting point of sulphur (444°), and the slight decomposition which occurs at that temperature, being accompanied by an action on the glass, may be due to that action. A minute quantity of oxygen seems to be liberated, and there is a very slight greying of the faintly yellow liquid; on cooling and dissolving, there is slight turbidity from the presence of silver, and a trace of nitrite can be detected. Only at a much higher temperature does the salt decompose with free effervescence, and then nitric peroxide accompanies the oxygen, and silver is deposited; even then, nitrite is present in the mass only in very small quantity at any time, there never being enough to remain undissolved when the nitrate is treated with a little water. This is sufficient, however, to show that the primary decomposition of silver nitrate by heat alone is into silver nitrite and oxygen, the instability of silver nitrite at much lower temperatures, although diminished by the presence of nitrate (Trans., 1871, 24, 85), accounting fully for its being found in such small quantity when the temperature is high, and for the production of nitric peroxide and silver instead. As determined by Carnelley, the melting point of silver nitrate is 217°.

The nitric oxide used for the experiments was prepared by the ferrous sulphate method, stored for 2 days in a glass gas-holder, and dried in its passage to the silver nitrate by a sulphuric acid column. At starting, the air in the drying apparatus and in the tube containing

the silver nitrate was expelled by carbon dioxide, the silver nitrate being heated in the gas, in order to dry it. Interaction between the silver nitrate and the nitric oxide was recognised by the reddening of the gas, and at the end of an experiment the gases were expelled by carbon dioxide before opening the tube.

Silver nitrate, when heated in nitric oxide, is strongly affected by it, being freely decomposed at a much lower temperature than that at which it decomposes when heated alone, the nitric oxide becoming oxidised. The action commences, perhaps, at 150°, but this is doubtful; at the melting point of the salt, it becomes marked, and at the boiling point of mercury considerable, but even at this temperature it is a long time before the decomposition is complete, the progress of the change gradually becoming slower. For some time, the products are silver nitrite and nitric peroxide, AgNO3 + NO = AgNO2+NO2, but very little silver is liberated, the nitrite being almost wholly preserved for a long time, through combination with the undecomposed nitrate. But when, as the nitrate becomes decomposed, the nitrite is no longer unprotected, it suffers decomposition, as usual, into silver and nitric peroxide; finally, nothing but silver remains.

Theoretically, it is quite probable that nitric oxide does not, after all, act directly on silver nitrate. For, making the allowable supposition that, to a minute extent, silver nitrate decomposes into silver nitrite and oxygen, at temperatnres much below that at which it does so sensibly, the nitric oxide may be regarded as being active by combining with this oxygen, and thus, by removing it, greatly hastening the spontaneous decomposition of the nitrate. This decomposition, thus assisted, and occurring at temperatures at which silver nitrite is comparatively stable in presence of nitrate, the nitrite remains, although at higher temperatures it decomposes almost as fast as it is formed from the nitrate. According to this theory, silver nitrate is not actually decomposed by nitric oxide, but only decomposes much more rapidly in its presence, in consequence of its interaction with one of the products of decomposition. For practical purposes, silver nitrate and nitric oxide may, however, be treated as acting on each other when heated together.

Nitric oxide has no action on sodium potassium or barium nitrate, even at the temperature of boiling sulphur.

Lead nitrate soon begins to decompose by heat alone, and nitric oxide seems to be without effect on its decomposition. According to Stas, lead nitrate begins to decompose somewhere above 200°; I find that, for its fairly free decomposition, a temperature not much below 400° is required. At the boiling point of sulphur even, the decomposition proceeds at such a moderate rate that only after 10 minutes heating does the salt show distinct signs of fusing. No nitrite is pro