undue heating of the glass balloon, which otherwise must be sunk in running water in order to keep it cool; and it exposes a very large and constantly renewed surface of soda to the nitrous fumes which are produced by the combination of the nitrogen and the oxygen, and so removes them as quickly as they are formed. It appears probable that the union results initially in the formation of nitric oxide, NO, which then unites partially with oxygen to form some nitrogen peroxide, NO_{2}. This is absorbed by the soda, giving a mixture of nitrite and nitrate of sodium, NaNO_{2} and NaNO_{3}. Working in this way, from 7 to 8 litres of mixed gases can be made to combine per hour. The rapidly alternating current is best obtained by the use of a transformer; and as the heating effect on the platinum terminals is very great, they must be made of stout rods. (2) To prepare a large quantity of argon by the absorption of atmospheric nitrogen by magnesium is a somewhat tedious process. The air must be first freed from oxygen by means of red-hot copper, and the atmospheric nitrogen collected in a gasholder. Long tubes of combustion-glass tubing, which stands a bright red heat without becoming deformed, are packed with magnesium turnings and heated to redness in long gas furnaces, such as are used in organic analyses; and through these the “atmospheric nitrogen,” dried by passage over soda-lime and phosphorus pentoxide, is then passed. The magnesium begins to glow at that end of the tube nearest the entrance, owing to its combination with nitrogen, and a hot ring is seen to travel slowly down the tube to the other end, marking the place where such combustion is in progress. The gas issuing from the tube is collected in a small gasholder. When one tube of magnesium is exhausted, another is substituted for it. Each tube is capable of absorbing about seven litres of nitrogen, so that to obtain a litre of argon about one hundred litres of “atmospheric nitrogen” must be employed, and about fourteen tubes of magnesium are required. M. Maquenne, who has prepared the nitrides of several metals, has stated that a mixture of lime and magnesium, yielding metallic calcium, is more easily manipulated than pure magnesium, owing to the absorption of the nitrogen at a lower temperature. The process has not been tried on a large scale, but if the temperature of combination of magnesium and nitrogen could be thus reduced, it would much facilitate the operation, for the greatest care has to be taken not to overheat the combustion tube, else it softens, and blows into holes. Porcelain tubes are attacked by the magnesium, and crack on cooling; and iron tubes are difficult to clean out.