"Out of reach of the flame of the fire, I took some black material like smoke."
Leeuwenhoek often worked by comparing structures and processes in a variety of related specimens. For the observations reported in this letter, he gathered his specimens from shops around Delft.
An apothecary, perhaps the passage that triggered Lister's concern:
Some time after this I discussed the making of salt of Carduus benedictus with an apothecary, who told me that the salt most of his colleagues sell as Sal Cardui benedicti was nothing but a mishmash of the burned stalks of various herbs, and that he had now freshly made, genuine Sal Cardui benedicti.
I have examined the salt of wormwood, as kept in bottles by the chemist.
Delft had only one armament factory:
On and above the iron doors at the mouth of the furnace in which cannons are cast, there is a white, ash-like substance which is formed there from the matter ejected from the furnace by the copper, the tin and the flame of the wood and there adheres or collects. I took some of this ash.
A Delftware factory:
Our potters use much tin and lead which they burn, in furnaces, to ashes. This process of burning, which they sometimes continue for ten to twelve days on end, so as to lay up a stock that will last them a long time, and avoid having to spend two or three days at it every month, is so injurious to the health of those engaged on the work that they are overcome after no more than 24 hours' attendance at the furnace, and then look as though they had been poisoned.
This means that a different man has to be put every day on the job of watching the furnaces and continually skimming off the ashes which accumulate on the surface of the molten lead and tin, and scooping it out of the furnace. This led me to collect the ash-like substance on the bricks close to the mouth of the furnace, licked by the flame which constantly spurts through the mouth of the furnace, and to put it in water and examine the latter when it had become clear.
From the chimney of the tin- and lead-furnace, about two feet from its mouth, out of reach of the flame of the fire, I next took some black material like smoke.
A soap factory:
I went to visit two soap boilers so that I might understand what use or purpose the potashes and soda serve in soap-making. They told me that it was impossible to make solid or thick soap from the strength or salt of potash or soda alone, but that it was necessary to mix lime-water with it and that only the latter solidifies or stiffens the soap. They also told me that the water or salt of lime burned from stone is best for the purpose, though they did not know why.
From the observations I have reported, however, we may well find reasons why.
Preview of the upcoming letter
In the previous Letter 43, Leeuwenhoek had written:
P.S. After concluding this, I opened a bitch, or female dog, and found in its womb, or let it rather be said in both tubes, a large quantity of the male seed of a dog, of which I shall treat further on a future occasion.
In this Letter 44, he wrote:
A few days ago I again acquired a bitch on heat which, I was told, had mated three or four times three days previously. I killed it and examined the womb, which was reasonably wide, and there found everywhere an enormous number of living animalcules, the male seed of the dog. I shall pursue these observations of mine and send you my notes thereon at a future date.