[35] If the compression be brought about slowly, so that the heat evolved succeeds in passing to the surrounding space, then the combination of the oxygen and hydrogen does not take place, even when the mixture is compressed by 150 times; for the gases are not heated. If paper soaked with a solution of platinum (in aqua regia) and sal ammoniac be burnt, then the ash obtained contains very finely-divided platinum, and in this form it is best fitted for igniting hydrogen and detonating gas. Platinum wire requires to be heated, but platinum in so finely divided a state as it occurs in this ash inflames hydrogen, even at -20°. Many other metals, such as palladium (175°), iridium, and gold, act with a slight rise of temperature, like platinum; but mercury, at its boiling point, does not inflame detonating gas, although the slow formation of water then begins at 305°. All data of this kind show that the explosion of detonating gas presents one of the many cases of contact phenomena. This conclusion is further confirmed by the researches of V. Meyer (1892). He showed that only a very slow formation of steam begins at 448°, and that it only proceeds more rapidly at 518°. The temperature of the explosion of detonating gas, according to the same author, varies according as to whether the explosion is produced in open vessels or in closed tubes. In the first case the temperature of explosion lies between 530°-606°, and in the second between 630°-730°. In general it may be remarked that the temperature of explosion of gaseous mixtures is always lower in closed vessels than when the detonating mixture flows freely through tubes. According to Freyer and V. Meyer, the following gases when mixed with the requisite amount of oxygen explode at the following temperatures: +----------------+---------------------+-------------------+ | -- | When flowing freely | In closed vessels | +----------------+---------------------+-------------------+ | H_{2} | 630°-730° | 530°-606° | | CH_{4} | 650°-730° | 606°-650° | | C_{2}H_{6} | 606°-650° | 530°-606° | | C_{2}H_{4} | 606°-650° | 530°-606° | | CO | 650°-730° | 650°-730° | | H_{2}S | 315°-320° | 250°-270° | | H_{2} + Cl_{2} | 430°-440° | 240°-270° | +----------------+---------------------+-------------------+