[ 3*2 ] 
It is remarkable that more water is in general re- 
quired to produce this change on the veficles of the 
Unman blood than on thofe of frogs, or ether amphi- 
bious animals ; and thofe of the amphibia require ftill 
more than thofe of fifli ; for the fubftance of thefe 
veficles being thicker and more coloured in man and 
in quadrupeds than in the amphibia, is therefore 
later in being diflolved in water; and being thinned; 
in fifh, it thence moft readily diffolves. Thofe who 
are defirous of repeating thefe experiments had beift 
begin with the blood of toads and frogs, whofe vefi- 
oles are large, and remain fome time without diffolv- 
ing in the water (when that is ufed with the above- 
mentioned precautions) ; by which means any one 
accuftomed to microfcopical experiments may rea- 
dily be fatisfied of thefe curious circumflances. 
From the greater thicknefs of the veficles in the 
human fubjedt, and from their being lefs tranfparent 
when made fpherical by the addition of water, and 
likewife from their being fo much fmaller than thofe 
of fifh or frogs, it is more difficult to get a fight of 
the middle particles rolling from fide to fide in the 
veficle, which has become round ; but widi a ftrong 
light *, and a deep magnifier, I have diftindlly feen 
it in the human fubjedt, as well as in the frog, toad, 
or fkate. 
Since water makes thefe particles round, and 
makes the dark fpoi in their middle difappear, it is 
•evident the red particles of the human blood are not 
•perforated, but that dark fpot is owing to fome- 
i-hifig elfe than a hole : and this is likewife con- 
* Thefe experiments were all made with day-light, in clear 
weather. 
r med 
