VOL. XXIII.] PHILOSOPHICAL TRANSACTION S. Q5 



as much aqua regia impregnated with gold, as was equal to something more 

 than a large pin's head; and set it over such a strong fire, that both the glass 

 and gold were near melting; then viewing the glass, I saw very plainly that a 

 small portion of the said gold was coagulated into branches, and that these co- 

 agulated particles were round knobs or bubbles, which were dispersed in great 

 numbers over the whole glass; in some places close together, in others thinner, 

 of different magnitudes, and many of them so small, that although I viewed 

 them through a good microscope, they in a manner escaped my view ; inso- 

 much that it is almost impossible to conceive, unless one has had ocular 

 proof of it, that such vast numbers of gold globules could proceed from so 

 small a quantity of matter ; and who knows how many more gold particles 

 must coalesce in melting, to compose one of these smallest globules. Now 

 though the minute gold particles could not be discovered, when surrounded 

 with aqua regia, yet we could easily see the shining gold colour of them all, 

 now (excepting only the very smallest globules) appearing like so many complete 

 round pellets. 



In my former letter I affirmed that the green stuff found on still waters, 

 which we also call duck weed, did not spring from the ground, but from seed 

 within it. But now in winter, when those ditches, in whicli I had seen much 

 of this green weed, were covered with ice, I saw but a very little of it, and 

 what was very small of leaf. On this I thought it might be possible in warm 

 weather for those green weeds to spring from the earth, though it did not 

 seed upon the ground, nor increase in size there. For supposing, as it really 

 happens, that these weeds in warm weather, by the rarefaction of those particles 

 of air in their vessels, are driven up to the superficies of the water, the same 

 weeds, by the contraction of the air in cold weather, will grow heavier than the 

 water, and consequently sink to the bottom, and according to the change of 

 the weather, will have their vicissitudes of emerging or subsiding. To be con- 

 vinced of this rising and falling of the green weeds, take a small glass-bubble, 

 such as is represented by fig. J 7, grst, leaving the small orifice at t open, for 

 the water to run in and out; and let this bubble be put into a vessel with water ; 

 and it will be found in summer that, by the dilation of the particles of air within, 

 the said bubble, it will rise to the surface of the water ; whereas the same air, 

 through its elasticity, being contracted in cold weather, will make room for the 

 admission of a mass of water so much heavier than itself, into the bubble, and 

 then it will subside. 



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