VOL. LXXI.] PHILOSOPHICAL TRANSACTIONS. 21 



dilute acid to a given weight of such solution, and then weighing it again ; for 

 as 21 is to 100, so is the weight lost, to the weight of mild alkali in such 

 solution. 



The specific gravity of mild and perfectly dry 4 times calcined fixed alkali, free 

 from siliceous earth, and containing 21 per cent, of fixed air, I found to be 

 5.0527. When it contains more fixed air, its specific gravity is probably higher, 

 except it were not perfectly dry : whence I inferred the specific gravity of this 

 alkali, when caustic and free from water, to be 4.234. 



From the weight of the aerial acid, in its fixed state, it happens, that fixed 

 alkalis, when united to it, are specifically heavier than when united either to the 

 vitriolic or nitrous acids. Thus Mr. R. Watson, in the Phil. Trans, for the 

 year 1770, p. 337, found the specific gravity of dry salt of tartar, including 

 siliceous earth, to be 2.761 : whereas the specific gravity of tartar vitriolate was 

 only 2.636, and that of nitre l.y33. The reason why nitre is so much lighter 

 than tartar vitriolate, is, because it contains much more water, and its union 

 with the alkali is less intimate. 



Impure vegetable fixed alkalis, such as pearl ash, pot ashes, &c. contain more 

 fixed air, as appears by the experiments of Dr. Lewis. Pearl ash, according to 

 Mr. Cavendish, contains 28.4 or 28.7 per cent, of fixed air. Hence, in lyes of 

 equal specific gravity with those of a purer alkali, the quantity of saline matter 

 will be more probably in the ratio of 28.4 or 2S.7 to 21 : but this surplus weight 

 is only fixed air. 



III. Account of the violent Storm of Lightning at East-Bourn, in Sussex, 

 Sept. ^7, 1780. Communicated by Owen Salusbury Brereton, Esq., F. R., 

 and A. S. p. 42. 



I am desired by my friend and neighbour James Adair, Esq., of Soho-square, 

 to communicate an account of Icie dreadful accident which happened to him and 

 his family at East-Bourn, in the county of Sussex, at Q o'clock in the morning, 

 on Sunday the 17th of September last, (1780). He rented a house which 

 stood by itself, built of various sorts of stone, 3 stories high, and facing the 

 sea, which was nearly south-east of it. The morning was very stormy, with 

 rain, thunder, and lightning; and just at 9 o'clock a horrid black cloud ap- 

 peared, out of which Mr. Adair saw several balls of fire drop into the sea 

 successively, as he was approaching the window in a one-pair of stairs room; 

 and very soon after, as he was standing at it with his hands clasped, and extended 

 open against the middle of the frame, a most violent flash of fire forced his 

 hands asunder, and threw him several yards on the floor on his back, with both 

 his legs upright in the air, which remained long so fixed. He was very sensible 

 of his situation all the time, but could not open his eyes nor speak; nor had he 



