VOL. XXX.] PHILOSOPHICAL TRANSACTIONS. 417 



of fact, for the truth of which they had so great an authority. But that the 

 globular part of the blood is specifically heavier than the serum, will appear from 

 the following experiments. 



Exper. I. I have several times cut off a small part of the crassamentum, 

 when by its adhesion to the sides of the porringer it has seemed to swim on 

 the surface of the serum, and have put it into another vessel filled with serum : 

 on which it has immediately sunk to the bottom. 



Exper. 1. When the coagulum has been buoyed up in the serum by the 

 bubbles of air adhering to its surface, I have separated a small part of it, 

 where those bubbles have been thickest, and put it into a glass of serum, in 

 which it has swum, as before. Then setting the glass upon the air-pump, 

 those bubbles burst after one another, as the receiver was exhausting; and the 

 air being again let into the receiver, the lump of crassamentum sunk to the 

 bottom of the glass. 



Exper. 3. I have often placed a drop of serum on a clean glass before a 

 microscope, in which I had dissolved a very small quantity of blood, and ob- 

 served, that when the glass was held in a perpendicular position, the blood- 

 globules subsided to the bottom of the drop; and inverting the glass, the 

 globules again descended through the serum to the bottom. I had the same 

 success with a small quantity of serum and blood in a capillary tube. And the 

 same thing has been long since observed by Mr. Leuwenhoeck. 



These experiments undeniably prove, that the crassamentum, or globular 

 part of the blood, is specifically heavier than the serum ; and consequently it 

 is by no means probable, that the blood globules are vesicles filled with air, 

 or any other fluid lighter than serum. And that they are not filled with any 

 sort of elastic fluid will appear from the following experiment. 



Exper. 4. In a small quantity of serum of human blood, 1 dissolved so 

 much blood, as that the globules might not lie too thick together, to hinder 

 their being seen distinctly. Then having lodged a small drop of this liquor on 

 the inside of a thin glass tube, I fitted the tube on to the air-pump, and placed 

 a microscope by it, so that I could see the blood-globules through the tube. 

 I then caused the tube to be exhausted, keeping my eye on the globules all the 

 time, in order to observe whether they dilated themselves as the air was with- 

 drawn ; \\xt could not perceive the least alteration, they appearing exactly of 

 the same size in the vacuum, as they had done before. Whereas had they 

 been filled with an elastic fluid, they would either have burst, or have been 

 dilated to at least 70 or 80 times their former magnitude. The stop-cock 

 being afterwards turned, and the air suffered to re-enter the tube, the blood- 

 globules still retained the same size as in vacuo, 



VOL. VI. 3 H 



