430 PHILOSOPHICAL TnANSACTlONS. [aNNO 1788, 



diminish during that time. It must be observed, that small spicule of ice always 

 ( ame over along with the decanted liquor ; and to this in all probability the new- 

 formed ice attached itself; for otherwise it is likely that no ice would have been 

 produced. » 



The following table contains the strength of the acids as determined before 

 they were sent to Hudson's Bay, and the quantity and strength of the decanted 

 and undecanted parts when they arrived at London, and the strength of the whole 

 mass as computed from thence. For the sake of uniformity, I have expressed 

 their strengths, like those of the nitrous acid, by the quantity of marble necessary 

 to saturate them, though I did not find their strength by actually trying how 

 much marble they would dissolve ; as that method is too uncertain, on account 

 of the selenite formed in the operation, and which in good measure defends the 

 marble from the action of the acid. The method I used was, to find the weight 

 of the plumbum vitriolatum formed by the addition of sugar of lead, and thence 

 to compute the strength, on the supposition that a quantity of oil of vitriol, suf- 

 ficient to produce 100 parts of plumbum vitriolatum, will dissolve 33 of marble; 

 as I found by experiment that so much oil of vitriol would saturate as much fixed 

 alkali as a quantity of nitrous acid sufficient to dissolve 33 of marble. It may be 

 observed that the quantity of alkali, necessary to saturate a given quantity of acid, 

 can hardly be determined with much accuracy, for which reason the foregoing 

 less direct method was adopted ; especially as the precipitation of plumbum vitrio- 

 latum shows the proportional strengths, which is the thing principally wanted, 

 with as great accuracy as any method I know. 



The undecanted part of N*^ 4 was divided into 2 parts ; viz. the less and the 

 more congealable part ; and it is the latter whose quantity and strength is given 

 in the last line. It is well known that oil of vitriol attracts moisture with great 

 avidity ; and some of these acids were much exposed to the air during the experi- 

 ments made with them, and may therefore be supposed to have attracted so much 

 moisture from the air, as might sensibly diminish their strength ; and this seems 

 actually to have been the case with some of them. But as the bottles were well 

 stopped, and as, except in one acid which was the most exposed to the air, the 

 strength of the whole mass comes out not much less than that determined before 

 the liquors were sent to Hudson's Bay, I imagine their strength could not sensibly 



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