VOL. XXIV.] PHILOSOPHICAL TRANSACTIONS. 207 



there can be no vacuum, the water will immediately succeed, and the piece of 

 coal must necessarily sink to the bottom. 



Now, that we may the better conceive the configuration of the pumice-stone, 

 I have placed several very small particles of it before a microscope. PI. 9, fig. 1, 

 represents a small particle of the fore-mentioned stone, in which we may per- 

 ceive the very small tubes, though by reason of their exceeding smallness those 

 cavities can be seldom seen ; some of the largest of them are represented between 

 G and H. I discovered also such exceedingly fine vessels, that several hundreds 

 of them are not equal to a single hair in the wool of sheep- The stone is of a 

 surprising and inconceivable figure; for whereas the canals or vessels in trees 

 and plants almost all appear very regular, some running perpendicular, others 

 horizontally, but the canals of this stone run so strangely, that one can hardly 

 form any idea of them, especially when viewing its particles as they lie in and 

 about those small cavities. 



Fig. 2 represents a small particle of the said stone, of which qiklmn lay near 

 a cavity that was in the stone, and wherein are described, as well as it is possible, 

 those small pipes of which that part is composed. Now as such a stone is con- 

 tinually growing, or, to speak more philosophically, the juices are always car- 

 ried up through those pipes, I suppose there happened some obstructions in 

 bringing those saps about lm, by which means those little branches, no and pci, 

 shot out into the cavity of the stone. 



I placed before another microscope a much smaller and thinner particle of the 

 stone, as appears in fig. 3; of which ksty lay near the cavity in the stone; and 

 there happening some obstructions in the pipes or vessels about stv, by which 

 the ascending juices were forced to alter their course, it occasioned an excres- 

 cence, which bent or protruded itself into the cavity of the stone, as at svwxy. 



On talking with the man where this stone is bought, he told me that it was 

 true the pumice-stone is found floating on the sea, but that there are whole 

 rocks of those stones, and that sponges grow upon them. Now, if what he 

 affirmed were true, I imagined that some common or sea-salt would be found in 

 some of the pores of this stone ; to satisfy myself in this, I took a piece of stone 

 about the size of the tip of my finger, and laid it on some charcoal till it was 

 glowing hot, and then threw it into clean rain water, imagining it would sink to 

 the bottom, but I was deceived in my expectation, for most of the water had 

 insinuated itself into the pores or pipes of the stone. After this trial, I took the 

 particles of the stone that were broken to pieces in the water, and laid them on a 

 clean piece of paper, and then put those pieces into water again, in order to 

 impregnate it with the salt particles that were in the stone ; I then placed some 

 of the water upon four distinct glass-plates, and when it was almost evaporated, 



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