“Here,” I said brusquely, taking him by the sleeve, “you’re not well.  We’ll call somewhere and get a drink.”

“Yes,” he said, again wiping his brow.  “I say ... did you hear?”

“Hear what?”

“Ah, you didn’t ... and, of course, you didn’t feel anything....”

“Come, you’re shaking.”

When presently we came to a brightly lighted public-house or hotel, I saw that he was shaking even worse than I had thought.  The shirt-sleeved barman noticed it too, and watched us curiously.  I made Rooum sit down, and got him some brandy.

“What was the matter?” I asked, as I held the glass to his lips.

But I could get nothing out of him except that it was “All right—­all right,” with his head twitching over his shoulder almost as if he had touch of the dance.  He began to come round a little.  He wasn’t the kind of man you’d press for explanations, and presently we set out again.  He walked with me as far as my lodgings, refused to come in, but for all that lingered at the gate as if loath to leave.  I watched him turn the corner in the rain.

We came home together again the next evening, but by a different way, quite half a mile longer.  He had waited for me a little pertinaciously.  It seemed he wanted to talk about molecules again.

Well, when a man of his age—­he’d be near fifty—­begins to ask questions, he’s rather worse than a child who wants to know where Heaven is or some such thing—­for you can’t put him off as you can the child.  Somewhere or other he’d picked up the word “osmosis,” and seemed to have some glimmering of its meaning.  He dropped the molecules, and began to ask me about osmosis.

“It means, doesn’t it,” he demanded, “that liquids will work their way into one another—­through a bladder or something?  Say a thick fluid and a thin:  you’ll find some of the thick in the thin, and the thin in the thick?”

“Yes.  The thick into the thin is ex-osmosis, and the other end-osmosis.  That takes place more quickly.  But I don’t know a deal about it.”

“Does it ever take place with solids?” he next asked.

What was he driving at?  I thought; but replied:  “I believe that what is commonly called ‘adhesion’ is something of the sort, under another name.”

“A good deal of this bookwork seems to be finding a dozen names for the same thing,” he grunted; and continued to ask his questions.

But what it was he really wanted to know I couldn’t for the life of me make out.

Well, he was due any time now to disappear again, having worked quite six weeks in one place; and he disappeared.  He disappeared for a good many weeks.  I think it would be about February before I saw or heard of him again.

It was February weather, anyway, and in an echoing enough place that I found him—­the subway of one of the Metropolitan stations.  He’d probably forgotten the echoes when he’d taken the train; but, of course, the railway folk won’t let a man who happens to dislike echoes go wandering across the metals where he likes.