I'm puzzled why the noted scientist Jan Ingenhousz's FRS reported study in the title has been almost universally interpreted by modern writers (in books, papers since ca. 1968, and now online resources) as Brownian motion being unwittingly observed in an inert substance. Thus giving him priority over Robert Brown by over 40 years because Ingenhousz's use of an inert material rules out any confusion with reporting a study of live microorganisms in motion.
The primary source is a short article of five pages, (quoted in part below). He is describing the benefits of what we now call a coverslip, a very thin glass plate to cover a subject on a microscope slide, for which he is duly credited as inventing. He illustrates the problems with uncovered liquids by a simple demonstration of fine charcoal suspended in an uncovered alcohol which evaporates so rapidly that convection currents cause "everything which is contained in it" to move i.e. implying of any size. This confuses any microscopy studies, in particular if assessing whether a moving small subject is alive or not.
I'm not a science historian, just an optical microscopy hobbyist, but do have a particular interest in and experience of Brownian motion demonstrations in both the liquid and gas phase.
Brownian motion in the liquid phase is only observed in suspended particles below ca. 4 - 5 microns and requires an optical microscope typically with at least 200 - 400X total optical mag to observe it e.g. a classic demonstration is fat globules in dilute milk. It is a distinctive jiggling motion of fat globules ca. 0.5 - 3 microns in diameter.
I've attempted comparable experiments to Ingenhousz with fine charcoal and uncovered ethanol at various temps. The convection currents are so large that particles up to 100 microns in size are moving and exactly as Ingenhousz describes, i.e. in a "confused, continuous and violent" way. Particles are in macroscopic motion almost two orders of magnitude greater in size than that in which Brownian motion is observed and which would overwhelm the chance of observing the distinctive microscopic Brownian motion. Only the low magnification of a 10X hand lens or low power on a microscope is needed to see this gross motion. We don't know his room temp but Ingenhousz does note that his "droplets" of alcohol have evaporated "in a few minutes" so it was rapid.
If Ingenhousz suspected that there was an underlying microscopic motion present in the smallest particles beyond the macroscopic motion of all sized particles caused by evaporation, wouldn't such an astute researcher (noted for his studies on photosynthesis) have tried the device he was advocating, the coverslip, to remove evaporative effects and also use water not alcohol to decrease evaporation rates?
After my own studies, I recently found one source who questions the near universal opinion that Ingenhousz was unwittingly observing and describing Brownian motion. This is Beale and Beale in their biography of Ingenhousz 'Echoes of Ingen Housz' 2011, pp.344-6 who remark when discussing one of the earliest interpretations of the work as Brownian motion:
"The justification [of Ingen Housz observing Brownian motion] is, however far less convincing than it is for Ingen Housz having devised the cover slip and is almost certainly an over-enthusiastic conjecture."
These authors also discuss a later part of Ingenhousz's article not quoted above on his described benefits of the use of a coverslip which they note offers "clinching evidence that Ingenhousz was not observing Brownian motion".
But these authors are the exception and don't understand why.
Extract from English translation source (my use of bold): P W van der Pas, 'The Discovery of the Brownian Motion', Scientiarum Historia, 1971, vol. 13, pp.27-35.
Primary French work and source for above translation. 'Nouvelles Expériences et Observations Sur Divers Objets De Physique' volume 2 by Jan Ingenhousz, 1789. pp.1-5.
A German edition was published in 1784.
[Pages 1-2] "REMARKS ON THE USE OF THE MICROSCOPE.
N.B. I have thought it appropriate to precede the following paper with these considerations which, although of no importance for those have frequently used the microscope, will at least be a guide to others.
I have often troubled my head about the problem to find a method to avoid the too rapid evaporation of a drop of water, or any other liquid, in which I wanted to observe the insects, and I know that other observers are plagued with the same problem. Even if one wishes to observe the shape and size of some of these corpuscules for even the short time during which such a droplet lasts in the focal point of a microscope, one must agree that, as long as the droplet lasts, the entire liquid and consequently everything which is contained in it, is kept in continuous motion by the evaporation, and that this motion can give the impression that some of these corpuscules are living, even if they have not the slightest life in them.
To see clearly how one can deceive one's mind on this point, if one is not careful; one has only to place a drop of alcohol in the focal point of a microscope and introduce a little finely ground charcoal therein, and one will see these corpuscules in a confused, continuous and violent motion as if they were animalcules which move rapidly around.
If the droplet is rather large, it has a convex surface which refracts the light more or less; if it is very small, it lasts hardly long enough to enable observing its contents at one's leisure."
... [page 4] "Under these films, the vaporisation is so slow that a droplet which would evaporate in a few minutes, hardly vaporises in the course of hours." ...
[continues to page 5.]