leaving fanciful streaks, some dotted, some curiously curved, as seen in fig. 193.

ANTIMONIURETTED HYDROGEN.

SbH3

610. Preparation by the action of nascent hydrogen upon antimonious chloride. This is done in an apparatus similar to that described for the preparation of arseniuretted hydrogen, using a solution of antimonious chloride. The gas burns with a flame resembling in colour that of the corresponding arsenic compound, and when a cold porcelain plate is depressed upon it a similar stain is produced. The antimony stain, however, is blacker than that produced by arseniuretted hydrogen.

611. The decomposition of antimoniuretted hydrogen by heat may be shown in a precisely similar way to that described for arsenic. In this case it will be noticed that the mirror is

formed in the tube much nearer to the flame than was the arsenic deposit, and also that it is deposited upon both sides of the heated spot.

612. To distinguish between the deposits of arsenic and antimony they may be treated with a clear solution of bleaching powder; the arsenic stain is dissolved, while the antimony remains unchanged. A striking way of showing this difference is to deposit the antimony upon a white plate, forming with the stains the symbol Sb in bold letters. The plate is then covered with arsenic stains so as to completely obliterate the symbol. When the solution of the hypochlorite is poured into the plate the arsenic stains are instantly dissolved away, leaving the letters printed with the antimony deposit entirely unacted upon.

DISSOCIATION

613. Dissociation of iodine.1 A small quantity of iodine is introduced into a test-tube, which is then drawn out and hermetically sealed. The tube is supported in front of a slit through which a beam of light is issuing, and an image of the slit projected upon the screen. On gently warming the tube by means of a Bunsen lamp the violet colour of the iodine vapour will be seen, but as the temperature is gradually raised the violet colour gives place to an intense and pure blue. As the tube cools again the violet tint once more makes its appearance. Care must be taken to avoid the use of too much iodine, or the blue colour will be too intense to allow of the passage of the light; it will be found that 25 gram for a test-tube of about 70 cubic centimetres capacity (i.e. a tube 15 centimetres by 2 centimetres) will give a good result. The tube should be suspended by a wire, so that the whole of it may be heated by brushing the flame over it.

1 See Table XXV. in the Appendix.

614. Dissociation of nitrogen peroxide.

A quantity of nitrogen peroxide is introduced into a flask, the neck of which has been drawn out, by delivering into it a small quantity of nitric oxide, and the flask is then sealed off. If the flask be cooled and placed in front of the lamp, a pale-yellow disc of light will

be seen upon the screen. On heating the flask the colour of the light will rapidly deepen until it has assumed such a dark-brown colour that it appears almost opaque. The flask may be conveniently cooled while in position by allowing a little ether to drop upon a piece of rag or blotting-paper placed upon the flask, and at the same time gently blowing the vessel to hasten the evaporation.

This change of colour may be seen almost equally well by supporting the flask over a white surface, and applying the heat.

615. Dissociation of ammonium chloride. A thin glass tube about 25 centimetres long and 20 millimetres bore has a piece of the stem of a clay tobacco pipe passed through it, and kept in position by two loosely fitting corks. A piece of pure crystallised ammonium chloride is placed in the tube near the middle;

FIG. 194.

at each end, immediately beyond the cork, is introduced a strip of blue litmus paper, which should be curled round so as to lie flat against the glass tube. One end of the tobacco pipe is connected by means of a piece of caoutchouc tube to a small pair of hand-bellows (fig. 194). Heat is applied to the fragment of

1 See Table XXVI. in the Appendix.

ammonium chloride, which at once begins to vaporise and to dissociate into the two gases, ammonia and hydrochloric acid, which diffuse in small quantities through the clay pipe stem. Owing to their difference in density, more ammonia passes through the porous stem in a given time than hydrochloric acid, so that the gas within the stem will contain a slight excess of ammonia; if a gentle stream of air be driven through the porous tube by means of the small bellows, and the issuing gas be allowed to impinge upon moistened turmeric paper, the presence of the free ammonia is at once made evident. For the same reason a slight excess of hydrochloric acid gas will be found in the air within the glass tube, and this will make itself evident by reddening the strips of blue litmus paper.

In this experiment it is better to use a pair of bellows, in order to sweep out the gases from the clay pipe, than a stream of hydrogen or other gas, as it is more advantageous to allow the ammonia to accumulate within the pipe and to drive it out at intervals with slight puffs.

FIG. 195.

PH3

HBT

616. Dissociation of phosphonium bromide. If the two gases, hydrobromic acid and phosphoretted hydrogen, be passed into a cooled vessel, they will combine and form crystals of phosphonium bromide. The phosphoretted hydrogen may be prepared by the action of phosphorus upon alcoholic potash (see Phosphoretted Hydrogen, Experiment 527), and collected in a gas-holder. The hydrobromic acid is best prepared by passing hydrogen and bromine over a heated platinum spiral (see Hydrobromic Acid, Experiment 218), as the current of gas can be regulated to any desired

rate.

The compound is conveniently collected by delivering the two gases into a flask, which should be cooled by being placed in a freezing mixture. If it be desired to preserve a quantity of

the compound as a specimen, the neck of the flask may be constricted as seen in the figure, and the tubes delivering the gases drawn out sufficiently fine to pass the constriction of the neck. Immediately the gases are mixed, a deposit of the phosphonium bromide begins to collect upon the sides of the flask, and in a short time a considerable quantity of the compound will be formed; the gases issuing from the exit tube may be allowed to escape into a convenient draught.

For the purposes of the experiment for which the phosphonium bromide is being prepared it will be found convenient to transfer small quantities of it to a number of small Ụ tubes, in which it can be sealed up and preserved until required for

use.

For this purpose the flask is removed from the freezing mixture, and a gentle stream of hydrogen is passed in by one of the two long tubes, the other one being closed by placing a pinch-cock upon the caoutchouc tube. One of the U tubes is connected with the exit tube, and is immersed in a freezing mixture. In this way a quantity of the compound will be condensed in the U tube, which may then be sealed up. When several of the tubes have been in this way prepared, the flask itself may be sealed up

constricted part.

at the

FIG. 196.

J

end. The long stem of a clay tobacco pipe is inserted into the tube, being fitted at each end by means of short pieces of caoutchouc tube, and being long enough to project one or two centimetres beyond the glass at each end (fig. 196). (As the stem of a

T