“The sal ammoniack first of all. You see it has produced particles of a pale sediment with no other apparent movement. Hmm?”

He handed it over for our inspection and we agreed that the other substance he was showing us produced the same result.

“Now, lixivium of tartar. A white cloud in the middle of the liquid, suspended equidistantly between the surface and the bottom.”

Again, the other substance behaved in the same fashion.

“Vitriol. A precipitation producing hard crystals forming on the side of the glass. A matching result again.”

“Salt.” He paused and examined the bowl carefully. “A slight creamy precipitation, but so slight you might miss it entirely.

“Violets. How pretty. A tincture of pale green. Most attractive. Two of them, in fact, as my chosen substance has produced the same result. I hope you are beginning to be convinced.”

He grunted at us in a satisfied fashion, then picked up a pinch of each powder and threw them separately onto the red-hot iron. We watched as they hissed, and gave off thick white fumes. Stahl sniffed at them, then grunted again. “No flame in either case. Slight smell of—what would you say?—garlic.”

He poured some water on the iron to cool it down, then casually tossed it out of the window, so it could lie on the ground and not poison us. “And there we are. We needn’t waste any more time. We have now run a total of six separate tests, and in each case the material you brought me in the brandy bottle reacts in the same way as this substance here. As an experimentalist of chemistry, gentlemen, I offer you my opinion that the material in the bottle is indeed unlikely not to be the same.”

“Yes, yes,” said Lower, finally losing patience. “But what is this other substance of yours?”

“Ah,” said Stahl. “The crucial point. My apologies for my little piece of drama. It is called white arsenic. Formerly used as a face powder by the more foolish and vanitous of women, and quite deadly in large quantities. I can prove that as well, for I did one other test.

“I have notes on all this, by the way,” he said, as he opened up two paper packages. “Two cats,” he said, picking the creatures up by the tails. “One white, one black. Both perfectly healthy last night when I caught them. I fed one two grains of the powder from the bottle, and the other the same amount of arsenic, both dissolved in a little milk. Both beasts are, as you see, quite dead.

“You’d better take both of them,” Stahl continued. “As you appear to have been delving into Dr. Grove’s intestines, you may want to have a look at theirs as well. You never know.”

We thanked him profusely for his kindness and Lower, gripping a tail in each hand, wandered off to the laboratory to anatomize the beasts.

“And what is your opinion of that?” he asked as we strolled along the High Street in the direction of Christ Church once more. Having established that the substance in the bottle was indeed arsenic—or, to be correct, that it consistently behaved like arsenic and never behaved unlike arsenic, so that it could reasonably be said to be arsenic-like—and, moreover, that a cat, when fed the substance, died in a manner very similar to the way that a cat fed with arsenic died, we were but one step away from an alarming conclusion.

“Fascinating,” I said. “Ingenious, and thoroughly satisfying in both method and execution. But I must reserve my final opinion until we have seen inside those cats. The syllogism you obviously have in mind is as yet incomplete.”

“Arsenic in the bottle, and Grove dead. But did arsenic kill Grove? You are quite right. But you suspect as well as I what conclusions the cats’ intestines will indicate.”

I nodded.

“We have everything to suggest Grove was murdered except for the one necessary factor.”

“Which is?” I asked as we trailed through the unfinished and unworthy entrance to the college and walked through the vast but equally unfinished quadrangle.

“We don’t have a reason, and that is the most important thing. It is Stahl’s problem with the why and the how, if you like. There is no point working out how it was done if we cannot say why. Fact of crime, and motive for committing it, are all that is needed—the rest is unimportant detail. Cui prodest scelus, is fecit. He who profits by villainy, has perpetrated it.”

“Ovid?”

“Seneca.”

“I believe,” I said a little impatiently, “that you are trying to say something.”

“I am. Just as Stahl can work out how chemicals mix with each other but has no idea why, so it is with us. We now know how Grove died, but we do not know why. Who might possibly have wanted to take so much trouble to kill him?”

“Causa latet, vis est notissima,” I quoted back, and was pleased for once to have foxed him.

“ ‘The cause is hidden…’? Suetonius?”

“ ‘But the effect is clear.’ Ovid again. You should know that one. We have at least established fact—if the cats are as we suspect. The rest is not in our field.”

He nodded. “Considering your method of reasoning about your blood, I find that strange. You have completely reversed yourself. In one case, you had a hypothesis and saw no need for prior evidence. In this case, you have the evidence and see no need for a hypothesis.”

“I could just as easily say that you have done the same. Besides, I do not dismiss the need for explanation. I merely say that it is not our job to formulate it.”

“That is true,” he conceded, “and maybe my discontent is vanity. But I feel that unless our philosophy can also answer the important questions as well, it is unlikely to change much. Both why and how. If science confines itself to how, then I doubt it will ever be taken seriously. Do you wish to attend the cats?”

I shook my head. “I would love to. But I should go and see my patient.”

“Very well. Perhaps you will join me at Boyle’s when you have finished? And this evening I have a great treat. We must not allow ourselves to become overburdened by experiment. Diversion is also necessary, I think. By the way, I wish to ask you something.”

“And that is?”

“Periodically, I make a circuit of the countryside; Boyle mentioned it when you arrived, if you remember. As I can’t practice in the town, I have to go outside to earn a little money, and I am very short at the moment. It is a Christian charity, and quite profitable, which is a fine combination. I set up a room on market days, hang out a sign and wait for the pennies to roll in. I was going to leave tomorrow. There is to be a hanging out Aylesbury way, and I want to bid for the corpse. Would you like to come? There will be more than enough work for both of us. You can rent a horse for a week, see the country. Can you pull teeth?”

I bridled at the idea. “Certainly not,” I said.

“No? It’s easy. I’ll be taking some pliers, and you can practice if you wish.”

“That’s not what I meant. I mean that I am not a barber. Forgive me for saying so, but I risk my father’s wrath in acting the doctor, and there are depths to which I will not sink.”

For once Lower was not offended. “You’re not going to be much use, then,” he said cheerfully. “Listen, I am going to towns of a few hundred souls at most. Villagers come from miles around, and they want full treatment. They want to be bled, purged, lanced, have their piles rubbed and their teeth out. This isn’t Venice, where you can send them to the barbershop next door. You’ll be the only properly trained person they will see for another year, unless some wandering charlatan passes through. So if you come with me, you leave your dignity behind, as I will. No one will see, and I promise not to tell your father. They want a tooth out, you reach for the pliers. You’ll enjoy it; you’ll never have such appreciative patients again.”

“What about my patient? I really don’t want to come back to find her dead.”

Lower frowned. “I hadn’t thought of that. But she doesn’t need any attention, does she? I mean, you can’t do much except wait and see whether she lives. And if you gave her more treatment that would spoil the experiment.”