204. Matches.-The use of P for matches depends on its low burning-point. Prepared wood is dipped into melted S, and the end is then pressed against a stone slab having on it a paste of P, KClO3, and glue. KNO3 is often used instead of KClO3. In either case the object is to furnish O to burn P. Matches containing KClO3 snap on being scratched, while those having KNO3 burn quietly. The friction from scratching a match generates heat enough to ignite the P, that enough to set the S on fire, and the S enough to burn the wood. Give the reaction for each. Paraffine is much used instead of S. Safety matches have no P, and must be scratched on a surface of red P and Sb2S3, or on glass.
205. Red Phosphorus.-Two or three allotropic forms of P are known, the principal one being red. If heated between 230 degrees and 260 degrees, away from air, the yellow variety changes to red, which can be kept at all temperatures below 260 degrees. Above that it changes back. Red P is not poisonous, ignites only at a high temperature, and is not phosphorescent, like the yellow. 206. Spontaneous Combustion of Phosphene, or Hydrogen Phosphide, PH3.
Experiment 114.—Put into a 20cc.flask 1 g. P and 50cc.saturated solution NaOH or KOH. Connect with the p.t. by a long d.t., as in Figure 44, the end of which must be kept under water. Pour 3 or 4cc.of ether into the flask, to drive out the air. It is necessary to exclude all air, as a dangerously explosive mixture is formed with it. Heat the mixture, and as the gas passes over and into the air, it takes fire spontaneously, and rings of smoke successively rise. It will do no harm if, on taking away the lamp, the water is drawn back into the flask; but in that case the flask should be slightly lifted to prevent breakage by the sudden rush of water. On no account let the air be drawn over.
The experiment has no practical value, but is an interesting illustration of the spontaneous combustion of PH3 and of vortex rings. What are the products of the combustion? An admixture of another compound of P and H causes the combustion.
Chapter XL.
ARSENIC.
Examine metallic arsenic, realgar, orpiment, arsenopyrite, arsenic trioxide, copper arsenite.
The compounds of arsenic are very poisonous if taken into the system, and must be handled with care.
207. Separation. Experiment 115.—Draw out into two parts in the Bunsen flame a piece of glass tubing 20cm long and 1 or 2cm in diameter. Into the end of one of the ignition tubes thus formed, when it is cool, put one-fourth of a gram of arsenic trioxide, As2O3, using paper to transfer it. Now put into the tube a piece of charcoal, and press it down to within 2 or 3cm of the AS2O3 (Fig. 45). Next heat the coal red-hot, and then at once heat the As203. Continue this process till you see a metallic sublimate- metallic mirror-on the tube above the coal. Break the tube and examine the sublimate. It is As. Heat vaporizes the As2O;3. Explain the chemical action. What is the agency of C in the experiment? Of As2O3? 2 As2O3 + 3 C = ?
208. Tests.-Experiments 115 and 116 are used as tests for the presence of arsenic.