William Molyneux demonstrated blood flow in a newt to the Dublin Philosophical Society

In October 1683, William Molyneaux founded the Dublin Philosophical Society.

Two years later, in a letter to the Royal Society of October 27, 1685, Molyneaux wrote to defend his priority:

In the first, he [Dr. George Garden] gives an Account of the Visible Circulation of the blood in the Water-Newt or Lacerta Aquatica; truely I am hearily glad, that this Learned and Ingenious Dr. has hit upon this Experiment; tis now above two years and an half, since I first Discovered this surprising appearance, and wrote a large account thereof May 12, 1683, as also of the whole Anatomy of this Animal, to my Brother, who was then at Leyden. And I have since that, shew'd it frequently, both on the out-side without Dissestion, and in the inwaard Vessels also, to several Curious Physicians and Philosophers, to their great satisfaction and admiration; particularly I exposed it first to our Society [Dublin Philosophical] May the 26. 1684. as appears by the following minute taken from out Registry.

Molyneaux then quoted from the minutes of the Dublin Philosophical Society that he had sent to the Royal Society after that demonstration. They were read into the Royal Society minutes on June 18, 1684.

Mr. Molyneux opened before the company a water-newt, which he takes to be the salamandra aquatica. In the body of this animal there are two long sacculi aerei, on which the blood vessels are curiously ramified: to these blood vessels applying a microscope, he shewed the circulation of the blood ad oculum as plainly as water running in a river, and more rapidly than any common stream.

In the 1685 letter, Molyneaux continued:

The same experiment I repeated again before them on the 2d of June following, and to those that had good observing Eys, the Circulation was as visible outwardly on the hands and toes, as in the Vessels within. But Certainly the Appearance in the Vessels on the two forementioned sacculi, with the beating, emptying & filling of the Heart, is most surprising to the beholder.

Molyneaux was not the first to observe blood flowing. It was another several years, fall of 1688, before Leeuwenhoek published his observation of the red blood cells in a fish's tail making the turn from arteries to veins via capillaries.

The image on the right accompanied Molyneaux's 1685 letter to the Royal Society.