Wrote Letter L-028 of 1675-12-20 to Henry Oldenburg about various omissions in previous letters, small animals in water, his theory of “globules”, and the aerometer he devised

December 20, 1675
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Text of the letter in the original Dutch and in English translation from Alle de Brieven. The Collected Letters at the DBNL - De Digitale Bibliotheek voor de Nederlandse Letteren

The original manuscript on two folio pages and one quarto page, written and signed by Leeuwenhoek, is preserved at the Royal Society (MS. 1844. Early Letters L1.16). Leeuwenhoek's red wax seal is still attached to the outer quarto page, which reads, "A: Monsieur / Monsieur Grubenol ' Acc Dec 25.75 / a. Londen".

Leeuwenhoek wrote this letter to Henry Oldenburg:

Notifying the Royal Society about an error in Letter L-022 of 11 February 1675 concerning experiments of evaporating vegetable infusions

live creatures in water; wine-vinegar

his invention of an areometer, although he saw that Robert Boyle had constructed a similar glass in an article in Philosophical Transactions (volume 10, page 340)

He described live creatures in water as well as his technique for examining nerves and blood cells. He added (my translation and emphasis):

I looked forward to another letter to learn of the opinion of the Gentlemen on my theses, for I expect to be contradicted since the speculations set forth ... will appear strange to some people. I will be greatly obliged if these objections are communicated to me.

Last summer I made many observations on various waters and discovered in most of them numerous very small animals, where some were unbelievably small and beyond the little animals that others have discovered in water.

He also explained the "globules" that he was seeing in almost everything he looked at, globules within globules.

Several times I wrote about bodies consisting of globules, but we must not imagine perfectly round globules, but a number of bladders of animals, perfectly round, filled with water, and those bladders lying one next to the other on the earth. These bladders will appear perfectly round to the eye, but the part of the bladder that rests on the earth will not be round; but the part that touches the earth will be flattened in conformity with the earth; and if a large number of these bladders were thrown into an empty barrel or packed tight therein, the round bladders would not maintain their shape, but the said bladders would adapt themselves and press together very closely, no empty spaces remaining between them. Therefore every one of them would have a different shape, being all the said bladders very flexible.

These bladders, which I imagine lying in a barrel, I call globules, for they resemble globules very closely, for the upper part of bodies, if these be soft, will always maintain their round shape however small they may be, likewise as the bladders that are uppermost in the barrel will be round. This is also very clearly visible in the grains of the roes of fishes.

He ended the letter:

I saw that Mr. Boyle had invented similar glasses and that the said gentleman had thought much farther than I did, ... which I admit is a very important question.

In a parenthetic aside, he noted:

to my regret I do not understand English and in this town there is nobody who is able to translate it into Dutch for me.