Chap. 2.] Red Vefides in the Blood. 87 



neutral fait to the water, and they remain in it without 

 any alteration in their ihape, and without difiblving. 



' Now, when it is considered that the blood of all 

 animals is filled with thefe particles, we muft believe 

 that they ferve fome very important purpofe in the 

 animal ceconomy ; and fince they are fo complicated 

 in their ftrufture, it is improbable that they fhould 

 be formed by mechanical agitation in the lungs or 

 blood -veffels, as has been fufpected, but probably 

 have fome organs fet apart for their formation. This 

 I fhall endeavour to prove, when I have explained 

 their ftructure a little more particularly, and men- 

 tioned the manner in which I exhibit it. I take the 

 blood of a toad or frog, in which they are very large ; 

 I mix it with the ferum of human blood to dilute it; I 

 find them appear all flat, fo they do in the blood- vefiels 

 of this animal, as I have diftinctly feen in the web 

 between its toes, whilft the animal was alive, and 

 fixed in the microfcope. Their appearance in thefe 

 animals is not unlike flices of cucumber. I next 

 mix a little of the blood with water, which immedi- 

 ately makes them all round, and then begins to diflblve 

 them whilft they are round. I incline the ftage of 

 the microfcope, fo as to make them roll down it; 

 and then \ can diftinctly fee the folid in the middle fall 

 from fide to fide, like a pea in a bladder. A neutral 

 fait added to them at this time brings them back to 

 their flat fhape ; but if the fait is not added, the water 

 gradually diflblves away the veficle, and then the little 

 fphere is left naked. Such is the compofition of thefe 

 particles. I have exhibited thefe experiments to a 

 confiderable number of my acquaintance, who all 

 agree in their being fatisfadory. 



c The microfcope I ufe is a fingle lens, and there- 

 fore as little likely to deceive us as a pair of ipectacles, 

 G 4 



