Sir Clinton rapidly explained the cause of his visit; and producing the box of darts, he handed one of them to the toxicologist. “I’m not sufficiently ignorant to expect you to tell me what this stuff is on the spur of the moment, Dr. Ardsley; but I’m really trusting to luck that you may be able to make a guess at what the thing might be. If you can do even that, it may be of great importance to us.” Ardsley took the dart and examined it for a moment or two. Then he put questions about the state of the bodies and the times, which Sir Clinton was able to furnish. “H’m!” he said at last. “I think, from what you say, that I might make a guess at it. It’s obviously one of these arrow-poisons or something of that sort; perhaps a strophanthus derivative or a member of the strychnos group.” “Can you give me anything more definite?” Sir Clinton demanded, rather anxiously. “Time’s the main factor with me just now. I know these vegetable things are the very devil to spot; but it’s honestly a matter of life or death, and I want something definite if you can give me it.” Ardsley frowned slightly as he examined the dart. “Can you spare this? I mean, to examine it, chemically—and otherwise. I can’t promise to let you have it back intact, you know.” “Give me information, that’s all I ask.” “Very good.” He paused for a moment. “You won’t want to let this out of your sight, I suppose. Then you’d better come along to my laboratory. Luckily I have a guinea-pig in stock.” He glanced under his eyebrows at Wendover. “You’d rather stay here, I should think, Wendover. You dislike vivisection. I’m only going to put a needle into the little beast—quite painless; but you needn’t come and get your feelings rasped.” It was phrased politely enough; but it was quite evident from the way in which it was said that Ardsley had no desire to let Wendover into his laboratory. Leaving the Squire to kick his heels, the toxicologist led Sir Clinton through the house to the research department. “We’d better see exactly what phenomena the poison produces, first of all. I’ll get the guinea-pig.” He washed some of the poison from the dart with liquid, and introduced the solution into a hypodermic syringe, by means of which he injected a minute amount of the fluid under the guinea-pig’s skin. “Dead already?” Sir Clinton asked in some astonishment. “It’s like a thunderbolt.” Ardsley had been experimenting on the animal and watching closely. His face showed that he had found something definite. “I think I can make a guess,” he said. “It happens to be something with which I’m fairly familiar. Let’s confirm it.” He made another extraction of the poison which he placed in a test-tube. To this he added a few drops of solution from a bottle which he took down from a shelf. “Sulphovanadic acid,” he explained. “Just watch.” On the addition of the reagent, the liquid in the test-tube turned black. “It ought to change to dark blue, and then to red after a time.”