Robert Hooke produced two experiments on blood and milk and read the latter part of the Letter L-067 of 1678-01-14 about phlegm and little animals in pepper water

February 3, 1678

Birch, History, vol. IV, pp. 379-80, 24 January 1678 (O.S.) in London:

Mr. HOOKE produced two experiments, which he had promised at the last meeting, being those which were mentioned in the first part of Mr. LEEWENHOECK's letter; viz. concerning the constituent parts of blood and milk; which were very plainly to be seen by making use of a small piece of looking-glass plate (instead of the usual foot of the microscope) whicb was very smooth and clear; and by spreading a little of the blood and milk on the top of it, and looking against the flame of a candle. From whence it appeared, that the blood consists of two substances, the one a containing liquor undetetmined and undistinguishable as to its parts, flowing about and incompassing the other, which consists of an infinite number of exceedingly small parts, which were plainly perceived to be globular: all which parts were very equal as to bigness, and were seen upon the turning of the microscope to move to and fro very swiftly and very freely, they seeming to cross one another very much, and to move confusedly, though all tending the same way.

In milk the like substances were very visible, only with this difference, that whereas the globular parts of the blood were all of very equal magnitudes, those of milk were extremely different. These were exceedingly white like little pearls, whereas those of the blood were red.

The latter part of Mr. LEEWENHOECK's letter was then read, wherein he gave an account of his observations shewn on flegm, in which he had discovered the same globules, as in the blood; but that the vesicles, as he conceived, of them had received some kind of corruption and greenness.

He added also his observations of some exceedingly small animals in pepper-water not one thousandth part so thick as an hair, but three or four times as long as thick. These shot very nimbly through the water, and the length of their shoot was about half a hair's breadth. In old pepper-water likewise he had found eels no thicker than the former, and but one hundredth part of the length of an eel in vinegar.