[ 15 ° ] 
precipitate or fall to the bottom, and then the fluid 
is faid to be faturated with the body It has diffolved ; 
yet a fluid, which is faturated with one body, may 
afterwards diffolve others of different kinds, and keep 
all their particles fufpended together. 
When any menflruum has entirely diffolved a body, 
it will continue as tranfparent as it was before. The 
caufe of which may be afligned from what Sir Ifaac 
Newton difcovered by experiments} that the parti- 
cles of bodies muft be of a certain fize, or bignefs,' 
to caufe any refledlion or refradion of the rays of 
light at their furfaces. From whence he gives the 
reafon why fome bodies are opake, and others 
tranfparent ; and he alfo obferves, that the mofl: 
opake bodies, fuch as metals, being diffolved in an 
acid menflruum, and thereby reduced to their ulti- 
mate and fmallefl particles, do not take away the 
tranfparency of the menflruum. 
Hence we may always know how to diflinguifli a 
folution from a mixture ; for if a body be reduced 
to powder, and thrown into a fluid that will diffolve 
it, and they are then fliaken fuddenly together, the 
fluid will continue fomewhat opake till the folution 
be effeded, or till what remains undiffolved falls to 
the bottom. For in this cafe the particles are not at 
firfl reduced to their fmallefl fize, as they are always 
in a folution. I think, therefore, we may confider 
the tranfparency of an heterogeneous fluid (or one 
that contains in it particles of another body) as the 
criterion of a true folution, and where that is want- 
ing, it is only a mixture, as when water and air ap- 
pear together in froth, or in a cloud, or a thick mifl, 
it is only a mixture of thofe bodies, and not a folution 
of either. 
This 
