It was merely a spacious house, with some innocent steam machinery in one end of it and some big porcelain pipes running here and there.

No, not porcelain--they merely seemed to be; they were iron, but the ammonia which was being breathed through them had coated them to the thickness of your hand with solid milk-white ice.

It ought to have melted; for one did not require winter clothing in that atmosphere: but it did not melt; the inside of the pipe was too cold.
Sunk into the floor were numberless tin boxes, a foot square and two feet long, and open at the top end.

These were full of clear water; and around each box, salt and other proper stuff was packed; also, the ammonia gases were applied to the water in some way which will always remain a secret to me, because I was not able to understand the process.
While the water in the boxes gradually froze, men gave it a stir or two with a stick occasionally--to liberate the air-bubbles, I think.

Other men were continually lifting out boxes whose contents had become hard frozen.