It is to Mr. Payen that wo are indebted for a complete study of the very important question of the accidents which are liable to occur in the preparation and the drying of pyroxy-line.

During the preparation, Mr. Payen has observed that when he pours the ntricacid or the mixture of nitrate of potash and sulphuric acid on the cotton, the temperature sometimes raises high enough to bring about the inflammation of the fibre, which is out of the liquid. Every chemist has observed that in operating by dipping the cotton into the acid, if the operation is done slowly, so that a part of the cotton is out of the acid, a particular reaction takes place, and gives a disengagement of nitrous vapors. Sometimes they have seen this reaction take place so rapidly that it provokes an effervescence, to such an extent, that the liquid is ejected more or less quickly from the vessel which contains it, and the cotton disaggregates, and is converted into a brown-reddish solution.

These two accidents are greatly to be apprehended, and can be stopped by throwing the whole into cold water, but it is better to avoid it by dipping the cotton completely into the liquor, in which it may stay more than 48 hours without inconvenience. These two accidents have not yet produced dangerous results; that is the reason why we only indicate them; but as all recorded accidents have occurred during the drying, we shall be more particular on the precautions to be taken in this operation. We have seen cotton explode at a temperature of 140°; how shall we explain such explosions when direct experiment shows that the pyroxyline, in order to fulminate, requires a more elevated temperature? Mr. Payen has examined this phenomenon, and he has demonstrated that currents of air, heated even moderately by stoves or furnaces, may produce explosions? Everyone has remarked that if gun cotton is placed on ordinary paper, and if that paper is kept above lighted coals, at a distance, such that the paper is scarcely burnt, and that the hand very easily supports the temperature of the current of air, and the deflagration is not long in being produced - then, according to Mr. Payen, it is very probable that, under these circumstances, currents of air or gases resulting from the combustion, keeping a temperature above 212° and even 856°, may inflame some particles of the cotton where they do not sensibly affect the skin.

To ascertain the truth of his theory, Mr. Payen kept for a long time the pyroxyline in contact with fine sheets of metals or porcelain heated by steam, and in no case has he ever seen inflammation produced; and he has several times proved that currents of air, heated by the intermediary of metallic plates or masonry works, inflame the cotton at a temperature not above 186°.

The consequence of these observations is, that in the oven used to dry the pyroxyline, hot air, heated by furnaces, stoves, etc., ought to be excluded, even if it was mixed with cold air. On the contrary, an oven, with a current of air heated by steam, fulfils all the conditions of safety, and by using steam, the oven can be heated without danger nearly up to 212°.

Mr. Gaudin has proposed a very good method of drying small quantities of pyroxy-line.

Through the room in which the cotton is, he passes a current of air, which has been desiccated by quicklime. This process is eco-nomical and perfectly safe, but is too tedious to operate on large masses. Mr. Payen has seen that the pyroxyline, prepared with raw cotton, heated on a mercury bath, requires a temperature higher than 892° to take fire.

When prepared with pure cellulose, it ex-plodes at 329°. Nitric paper, according to this chemist, has an explosive property weaker than the cotton.