22 PHILOSOPHICAL TRANSACTIONS. [aNNO 1785. 



ticated air as must unite to it in order to reduce it into acid, that is, 2 or 3 times 

 its bulk, would disappear, and no more; so that the whole diminution could not 

 exceed 3 or 4 times the bulk, of the phlogisticated air: whereas, if the diminu- 

 tion proceeded from the burning of the inflammable matter, the purer the de- 

 phlogisticated air was, the greater and quicker would be the diminution. The 

 result of the experiments was, that when dephlogisticated air, containing only 

 ^V o( its bulk of phlogisticated air, that being the purest air I then had, was 

 confined between short columns of soap-lees, and the spark passed through it 

 till no further diminution could be perceived, the air lost -5V0 of its bulk; which 

 is not a greater diminution than might very likely proceed from the first-men-» 

 tioned cause; as the dephlogisticated air might easily be mixed with a little com- 

 mon air while introducing into the tube. 



When the same dephlogisticated air was confined between columns of distilled 

 water, the diminution was rather greater than before, and a white powder was 

 formed on the surface of the quicksilver beneath ; the reason of which probably 

 was, that the acid produced in the operation corroded the quicksilver, and formed 

 the white powder; and that the nitrous air, produced by that corrosion, united 

 to the dephlogisticated air, and caused a greater diminution than would other- 

 wise have taken place. When a solution of litmus was used, instead of distilled 

 water, the solution soon acquired a red colour, which became paler and paler as 

 the spark was continued, till at last it was quite colourless and transparent. The 

 air was diminished by almost half, and I believe might have been still further 

 diminished had the spark been continued. When lime-water was let up into the 

 tube, a cloud was formed, and the air was further diminished by about -f. The 

 remaining air was good dephlogisticated air. In this experiment therefore the 

 litmus was, if not burnt, at least decompounded, so as to lose entirely its purple 

 colour, and to yield fixed air; so that, though soap-lees cannot be decompounded 

 by this process, yet the solution of litmus can, and so very likely might the so- 

 lutions of many other combustible substances. But there is nothing, in any of 

 these experiments, which favours the opinion of the air being at all diminished 

 by means of phlogiston communicated to it by the electric spark. 



XXIV. Account of the Measurement of a Base on Hounslow-heath. By Major 

 General William Roy^ F. R. S., and A. S. p. 385. 

 The rise and progress of the rebellion which broke out in the Highlands of 

 Scotland in 1745, gave occasion to commence government surveys in that part 

 of the island. These were conducted by Lieut. Gen. Watson, a military engi- 

 neer, and chiefly under him executed by our author, then a subaltern officer in 

 the same corps. Though this work, which is still in manuscript, says Gen. R., 

 and in an unfinished state, possesses considerable merit, and perfectly answered 



