3o8 DEVELOPMENT OF HEAT. chap.
temperature, and the velocity is also much greater. During the extremely short time of heating, the gas pressure cannot come into equilibrium with that of the surroimding atmo- sphere, and it is assumed that the pressure of the gas stands in about the same ratio to that of the surrounding atmosphere as the corresponding absolute temperatures do to each other, t.6. about 20 : 1. After the short heating a sudden cooling takes place, so that the products of the reaction are pre- vented from passing back into the original condition during the cooling interval. The conditions striven after by Sainte Claire-Deville and his pupils by other methods ["the hot and cold tube "] (i9), are in these cases fulfilled to a large extent, namely, heating the substance to a very high degree, and suddenly cooling, so that further reaction with total decomposition is prevented.
Besides the electrothermic process, others of a tnily electrochemical character take place. In 1849, Perrot showed that a series of sparks from an induction machine can electrolyse water vapour so that oxygen collects at the anode and hydrogen at the cathode, and indeed m the proportions required by Faraday's law. This observation has been recently confirmed by Ludeking {W) and by J. J. Thomson {/^1\ An electrothermic decomposition also takes place so that electrolytic gas (a mixture of hydrogen and oxygen) is produced at both poles. The electrothermic evolution of electrolytic gas is often much greater than the electrolytic, and can, of course, be distinguished from this.
I (2:^) have shown that gases are often electrolytically dissociated, as in the case of the vapours produced firom alkali salts in a Bunsen burner. All salts of the same metal conduct equally well; probably on account of the large amount of water vapour present the salts are as good as completely converted into hydroxides. With respect to con- ductivity, the series is : thallium, lithium, sodium, potassium, rubidium, and caesium, of which the last is the best conductor. The rubidium and caesium (hydroxide) vapours are so strongly dissociated that their conductivity at extreme dilution can
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