VOL. XXXIX.] PHILOSOPHICAL TRANSACTIONS. 3i 



through its porous substance, as in sugar, salt, filtering paper and sponge. 

 This immediately suggested the hint, whether it might not be used instead of 

 the Mexican filtre to strain water. He accordingly made a hollow in a little 

 bit of it, and on pouring water into it, it strained very fast through the pores. 

 He then took the tophaceous tubes of osteocoUa, and stopping one extremity, 

 he poured water on it; when it transuded very fast through its porous substance. 

 He also recollected that he had a sponge for several years, which when he lived 

 at the Caroline bath, he had put in a pipe that conveyed the hot waters, and 

 by this means the sponge being incrustated with the ochre, which the hot 

 waters carry along with them and deposit in their passage, degenerated into 

 a tophus: he made a pit in the sponge, and filled it with water, when it ran 

 very fast through it. On this he resolved to make trial with a tophus of the 

 hot bath, of which he had a pretty large piece ; and for this purpose he had it 

 hollowed into a mortar, to see whether the water would pass through that 

 dense and solid stone: and it answered his expectation; as the water strained 

 through in the same manner as through the Mexican filtre and other tophi; 

 but by reason of the density of the stone, slower than through more porous 

 stones. 



Hence the Dr. thought, that the tophus of the hot baths is generated 

 from the water depositing its ochre, in flowing through the pipes, and is in- 

 sensibly concreted ; in like manner might the sea, beating on the rocks, deposit 

 saline earthy particles, from whose successive concretion this stone is gene- 

 rated, and rather grow on the rocks, than like rock-mushrooms, spring from 

 them. But considering the remarkable density of the hot bath tophus, through 

 which water filtrates, he had a mind to try the same experiment with the com- 

 mon stone used in building. The success answered expectation : for, a mortar 

 made of such stone served instead of the Mexican filtre, the water straining 

 equally clear through both. The water strained in this manner acquired at 

 first an earthy taste, which yet on repeated filtration it lost. 



As to the purifying quality of these filtres, the Dr. does not deny, but that 

 muddy and slimy waters may, by straining through such stones, become clear 

 and pellucid ; because these impurities do not dissolve in the water or inti- 

 mately incorporate with it, but only float in it. But besides these, no other 

 waters can by any means become purer, as he learned from repeated expe- 

 riments, both with the filtre from Holland, and with those made from the 

 tophus of the Caroline hot baths and common stone, on several kinds of river 

 and spring water; and with an hydrometer examining their weight both before 

 and after filtration, he found little or no difference. , , 



A -lol ?ii 



