288 A LETTER 



I find that the blood of all animals contains vesicles of this 

 sort. In human blood there are millions of them, and they 

 give to it the red colour. But in insects they are white, and 

 less numerous in proportion than in man and quadrupeds. As 

 they are flat in all animals, I suspect that shape is a circum- 

 stance of importance ; but it can be altered by mixture with 

 different fluids. And I find that it is by a determinate quantity 

 of neutral salt contained in the serum that this fluid is adapted 

 to preserving these vesicles in their flat shape. For, if they be 

 mixed with water they become round, and dissolve perfectly ; 

 but add a little of any neutral salt to the water, and they 

 remain in it, without any alteration of their shape, and without 

 dissolving. 



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

 filled with these particles, we must believe that they serve some 

 very important purpose in the animal economy ; and since they 

 are so complicated in their structure, it is improbable they 

 should be made by mechanical agitation in the lungs or blood- 

 vessels, as has been suspected, but probably have some organs 

 set apart for their formation. This I shall endeavour to prove, 

 when I have explained their structure a little more particularly, 

 and mentioned 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 serum of human blood to dilute it; I find them 

 appear all flat ; so they do in the blood-vessels of this animal, 

 as I have distinctly seen in the "web between its toes, whilst 

 the animal was alive, and fixed in the microscope. Their ap- 

 pearance in these animals is not unlike slices of cucumber. I 

 next mix a little of the blood with water, which immediately 

 makes them all round, and then begins to dissolve them whilst 

 they are round. I incline the stage of the microscope, so as 

 to make them roll down it, and then I can distinctly see the 

 solid in the middle fall from side to side, like a pea in a bladder. 

 A neutral salt added to them at this time brings them back 

 to their flat shape ; but if the salt be not added the water 

 gradually dissolves away the vesicle, and then the little sphere 

 is left naked. Such is the composition of these particles. I 

 have exhibited these experiments to a considerable number of 

 my acquaintance, who all agree in their being satisfactory. 

 The microscope I use is a single lens, and therefore as little 



