"There, Tom Swift, it ought to work now!"

Josephus Baxter held up a large laboratory test tube, in which seethed and bubbled some strange mixture, turning from green to purple, then to red, and next to a white, milky mixture.

"Do you think you've hit on the right combination?" asked the young inventor, whose latest idea, the plan of fighting fires in skyscrapers from an airship as a vantage point, was taking up all his spare moments.

"I'm positive of it," said Mr. Baxter. "I've dabbled in chemicals long enough to be certain of this, even if I can't get on the track of the missing dye formulae."

"That certainly is too bad," declared Tom. "I wish I could help you as much as you have helped me."

"Oh, you have helped me a lot," said the chemist. "You have given me a place to work, much better than the laboratory I had in the old fireworks factory of Field and Melling. And you have paid me, more than liberally, for what little I have done for you."

"You've done a lot for me," declared Tom. "If it had not been for your help this chemical compound would not be nearly as satisfactory as it is, nor as cheap to manufacture, which is a big item."

"Oh, you were on the right track," said Mr. Baxter. "You would have stumbled on it yourself in a short time, I believe. But I will say, Tom Swift, that, between us, we have made a compound that is absolutely fatal to fires. Even a small quantity of it, dropped in the heart of a large blaze, will stop combustion."

"And that's what I want," declared Tom. "I think I shall go ahead now, and proceed with the manufacture of the stuff on a large scale."

"And what do you propose doing with it?" asked Mr. Baxter.

"I'm going to sell the patent and the idea that goes with it to as many large cities as I can," Tom answered. "I'll even manufacture the airships that are needed to carry the stuff over the tops of blazing skyscrapers, dropping it down. I'll supply complete aerial fire-fighting plants."

"And I think you'll do a good business," said the chemist.

It was the conclusion of the final tests of an improved chemical mixture, and the reaction that had taken place in the test tube was the end of the experiment. Success was now again on the side of Tom Swift.

But when that has been said there remains the fact that it was just the other way with the unfortunate Mr. Baxter.

Try as he had, he could not succeed in getting the right chemical combination to perfect the dye process imparted to him by his late French friend. With the disappearance of the secret formulae went the good luck of Josephus Baxter.

He had worked hard, taking advantage of Tom's generosity, to bring back to his memory the proper manner of mixing certain ingredients, so that permanent dyes of wondrous beauty in coloring would be evolved. But it was all in vain.

"I know who have those formulae," declared the chemist again and again. "It is those scoundrels, Field and Melling. And they are planning to build up their own dye business with what is mine by right!"

And though Tom, also, believed this, there was no way of proving it.

As the young inventor had said, he was now ready to put his own latest invention on the market. After many tests, aided in some by Mr. Baxter, a form of liquid fire extinguisher had been made that was superior to any known, and much cheaper to manufacture. Veteran members of fire departments in and about Shopton told Tom so. All that remained was to demonstrate that it would be as effective on a large scale as it was on a small one, and big cities, it was agreed, must, of necessity, add it to their equipment.

"Well, I think I'll give orders to start the works going," said Tom, at the conclusion of the final test. "I have all the ingredients on hand now, and all that remains is to combine them. My airship is all ready, with the bomb-dropping device."

"And I wish you all sorts of luck," said Mr. Baxter. "Now I am going to have another go at my troubles. I have just thought of a possible new way of combining two of the chemicals I need to use. It may be I shall have success."

"I hope so," murmured Tom. He was about to leave the room when Koku, the giant, entered, with a letter in his hand. The big man showed some signs of agitation, and Tom was at once apprehensive about Eradicate.

"Is Rad--has anything happened--shall I get the doctor?"

"Oh, Rad, him all right," answered Koku. "That is him not see yet, but mebby soon. Only I have to chase boy, an' he make faces at me--boy bring this," and the giant held out the envelope.

"Oh!" exclaimed Tom, and he understood now. Messenger boys frequently came to Tom's house or to the shops, and they took delight in poking fun at Koku on account of his size, which made him slow in getting about. The boys delighted to have him chase them, and something like this had evidently just taken place, accounting for Koku's agitation.

"This is for you, Mr. Baxter, not for me," said Tom, as he read the name on the envelope.

"For me!" exclaimed the chemist. "Who could be writing to me? It's a big firm of dye manufacturers," he went on, as he caught a glimpse of the superscription in the upper left hand corner.

Quickly he read the contents of the epistle, and a moment later he gave a joyful cry.

"I'm on the trail! On the trail of those scoundrels at last!" exclaimed Josephus Baxter. "This gives me just the evidence I needed! Now I'll have them where I want them!"