VOL. LXXVIII.] PHILOSOPHICAL TRANSACTIONS. 4ig 



XL Experiments and Observations relating to the Principle of Acidity, the 



Composition of Water, and Phlogiston. By Joseph Priestley, LL. D., F. R. S. 



p. 147. 



That water consists of two kinds of air, dephlogisticated and inflammable, is 

 now, I believe, generally admitted as one of the most important, and best as- 

 certained, doctrines in chemistry. My own experiments having seemed to 

 favour it, I made no difficulty of receiving it myself: but having, at the time of 

 the publication of the last volume of my experiments, found that, in decom- 

 posing the two kinds of air above mentioned by the electric spark, I got much 

 less water than I expected, and, instead of it, a dark- coloured vapour, not easily 

 condensed, I could not help concluding that something yet remained to be in- 

 vestigated with respect to this subject, and determined, at a proper opportunity, 

 to resume my inquiries into it. It is not necessary here to detail these ex- 

 periments, which we think were also re-printed, with additions by the author 

 in a separate tract. 



• But from these experiments are inferred the following conclusions. The near 

 coincidence of the results of these different experiments is remarkable, and 

 makes it almost certain, that no marine acid is retained in the terra ponderosa 

 that has been dissolved in it, after exposure to a red heat ; that the generation of 

 the fixed air carries off part of the water in the menstruum ; and that this part 

 of the weight is about one-half of the whole. It is observed also, that the sup- 

 position of water entering into the constitution of all the kinds of air, and 

 being as it were their proper basis, that without which no aeriform substance 

 can subsist, which the preceding experiments render in a high degree probable, 

 makes it unnecessary to suppose that water consists of dephlogisticated air and 

 inflammable air, or that it has ever been either composed or decomposed in any 

 of our processes. That water is decomposed when inflammable air is procured 

 from iron by steam, is not probable; since the inflammable principle may very 

 well be supposed to come from the iron, and the addition of weight acquired by 

 the iron may be ascribed to the water which has displaced it. Also when the 

 scale of iron, or finery cinder, is heated in inflammable air, it gives out what it 

 had gained, viz. the water. 



The most plausible objection to this hypothesis is, that iron gains the same ad- 

 dition of weight, and becomes the same thing, whether it be heated in contact 

 with steam, or surrounded by dephlogisticated air. But from the preceding ex- 

 periments it appears, that by far the greatest part of the weight of dephlogisti- 

 cated air is water; and the small quantity of acid that is in it may well be sup- 

 posed to be employed in forming the fixed air, which is always found in this 

 process: for that there is one common principle of acidity, and that all the acids 

 are convertible into one another, at least the nitrous acid into fixed air, is by no 



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