aOO PHILOSOPHICAL TRANSACTIONS. [aNNO 1692-3. 



it is a wonder to see any one of them free." And again, where he speaks of 

 he African kingdom or empire of Monomotapa, he has this memorable pas- 

 sage, " The natives never live long, by reason of the badness of the waters in 

 the country : for at the age of 25 they begin to be dropsical, so that it is a 

 wonder if any among them live above 40 years." What M. Tavernier delivers 

 being taken for granted, it seems very probable, that these people maybe much 

 relieved, and be brought to live as long as other nations, if they had a compen- 

 dious way to provide themselves plentifully with water, whose crudity is cor- 

 rected, its grosser and heavier parts separated, and its brackishness destroyed 

 by the fire, as its action is regulated and helped by their invention. 



The experiment mentioned in this paper was tried at a meeting of the Royal 

 Society, Feb. 17, l^Qi, by Dr. Sloane, with a success answerable to the asser- 

 tions of the hon. author, and that I or 2 drops of spirit of salt, mixed with 

 common water, would be by the same method discovered. 



At another meeting of the Society, on the 2d of March following, Dr. Hook 

 read a lecture concerning another method of his own, for the discovering the 

 smallest quantity of salt contained in water, from a principle of hydrostatics ; 

 and he produced the apparatus which he had prepared to exhibit the same be- 

 fore the persons then present ; and it was there shown, that the instrument he 

 applied to that scrutiny did very evidently discover the mixture of a 2000th 

 part of salt added to common water, and it seemed that it would easily have de- 

 tected half that quantity of salt added to an equal quantity of water. 



The method was by means of a large poise of glass, somewhat of the shape 

 of a bolt-head, the ball of which was about 3 inches diameter, but its stem or 

 neck not above half a line, or a 24th part of an inch ; this was so poised by red 

 lead put into it, as to make it but a little heavier than fresh water. Then this 

 poise was suspended by the small stem to the end of a slender and nice beam, 

 and being not overcharged with weight, would turn with a small part of a grain. 

 This beam was hung on a steady frame, and the poise hanging at one end of it 

 covered with the water to a certain mark or division made on the small neck, it 

 was counterpoised by some small weights put into the opposite scale of the 

 balance. Then a 2000th part of the water's weight was taken of common salt, 

 and put into the whole 2000 parts of the water, which by being stirred soon 

 dissolved. Then it was manifestly seen that near half an inch more of the neck 

 emerged out of the water so seasoned, than before the 2000th part of salt was 

 dissolved in it. This was only one use of this method of discovering very 

 small alterations in the constitutions of bodies, the same author having long 

 since, viz. Oct. 25, 1677, shown to the Society a method of discovering divers 

 alterations much more minute, viz. to the l07000th part of its weight. 



