It was some months after the beginning of the experiments when one of the junior chemists came to Tom with the announcement that he had discovered what he thought might be the very substance required. A German firm, Schlagintwert & Co. of Düsseldorf, had placed on the market a few months earlier a powder which, used as a solution, was highly valuable in preparing photographic plates. The exact ingredients of the powder were unknown, although by analysis it had been found to consist of nitrate compounds; but the buyer was warned by a label that it should not be exposed to great heat owing to the danger of explosion. It had occurred to the chemist to mix a little of this powder with petrol. The result was a paste which dried hard, but gave off almost infinitesimal particles of a highly explosive nature, when floating in an air chamber, though the paste itself was not explosive either under heat or shock. Tom was delighted with the discovery, and at once proceeded to construct an engine suitable to the peculiar properties of the composition. In building the motor he adapted the principle of the turbine to airship navigation. A powerful fan drove the current of air through a number of perforated aluminium plates covered with the paste. The resultant mixture of air and explosive particles passed into the explosion chamber, the intake being controlled by automatic valves connected with the turbine. The explosion of the mixture was brought about by a sparking plug connected with a small electric battery, the sparking being controlled by a cam on the shaft. At each explosion, the gas generated was forced at an enormous pressure through the turbine to the right of the explosion chamber, thus driving the propeller fixed on the shaft. Tom made his fan serve a double purpose, not only to drive air through the aluminium plates, but to send a current round a jacket on the outside of the turbine and thus keep the latter cool. This was a highly necessary arrangement owing to the enormous heat generated. From the first, indeed, the difficulty of cooling the turbine was the most serious with which he had to grapple. It required months of experiment before the engine could be worked for more than two or three minutes at a time. Gradually, however, by increasing the power of the fan, and constructing the turbine casing and blades of an alloy specially adapted to resist the effects of intense heat, this difficulty was to a great extent overcome.