VOL. XLIX.] PHILOSOPHICAL TRANSACTIONS. 641 



salt, then the salt would be the 480th part of the ounce of water. But as there 

 is 9 times less salt, therefore the proportion ofthe quantity of spirit of salt will 

 be but the 4320th part. And how much less must be the proportion of salt in 

 these distilled waters, which is not sufficient to make a sensible impression on 

 solution of mercury, and but a faint one on much diluted solution of silver ? 

 Such distilled sea-water will not therefore probably be unwholesome ; almost all 

 spring-waters have some degree of salt in them : but if there were more of the 

 spirit of salt, a very small quantity of pot-ash, or pearl-ashes, or salt of tartar, 

 combined with it, will turn it into common salt, the quantity of which would be 

 extremely little. 



Since double the usual quantity of vapour may by way of ventilation be carried 

 off, common salt may thus be made much sooner, cheaper, and better ; because, 

 as there is much less fire used, so proportionably, less of the fine acid spirit of 

 the salt, in which its virtue consists, will be evaporated away : for it is well 

 known that the salt is best, which has undergone the least action of fire in 

 making. This more speedy method of evaporating will also be useful, in making 

 many other evaporations ; as in making pot-ash, &c. 



LJ^. On the Great Benefit of Fentilators in many Instances, in Preserving the 

 Health and Lives of People, in Slave and other Transport Ships. By Stephen 

 Hales, D.D., F.R.S. p. 332. 



Captain Thomson, of the Success frigate, in a letter to Dr. Hales, dated 

 London, Sept. 25, 1749, says, " that during the ventilation, the lower deck 

 hatches were commonly kept close shut ; by which means the air was drawn 

 down into the hold, from between the decks, through the seams of the ceiling, 

 along the timbers of the ship ; by which means they found the foul air soon 

 drawn off from between decks. Their rule for ventilating was for half an hour 

 every 4 hours : but when the ventilating was sometimes neglected for 8 hours 

 together, then they could perceive, especially in hot weather, a very sensible 

 difference by that short neglect of it ; for it would then take a longer time to 

 draw off the foul air. Their general rule was, to work the ventilators till they 

 found the air from them sweet. All agreed that they were of great service ; the 

 men being so sensible of the benefit of them, that they required no driving 

 to work that, which they received so much benefit by. They found this good 

 effect from verrtilation, that though there were near 200 men on board, for al- 

 most a year, yet he landed them all well in Georgia, notwithstanding they were 

 pressed men, and drawn out of jails, with distempers upon them. This is what 

 he believes but few transports, or any other ships, can boast of; which he im- 

 putes to the benefit received by the ventilators. It is to be remarked, that the 

 crew of this ship, which lay wind-bound for 4 months, with the expedition fleet 



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