relative to Black, Watt, and Cavendish. 509 



been over and over again mis-stated, even by eminent chemi- 

 cal writers. Those however, who are better occupied in ma- 

 king scientific discoveries than in reviewing them, may be 

 excused, if they appear to be often less exactly acquainted 

 with the opinions of others than with their own, so far at least 

 as we can fully acquit them of desiring to exalt their own 

 views, or the views of a particular sera, or a favourite author, 

 bjfc underrating all that has gone before. 



The mistake in this case has certainly in great measure 

 arisen from the circumstance, that the inquiries of Hales were 

 directed more to the generic and physical properties of gases, 

 than to their specific and chemical distinctions. He calls 

 "airs generated in effervescences" — "true permanent air"; 

 he has been supposed to mean that they are true atmospheric 

 air; his real meaning was — that they are true elastic fluids, 

 and, with the same permanence of constitution, possess the 

 same elastic force as common air. This important fact had 

 been before announced by Mayow, but was first ascertained 

 with precision by Hales. " That I might," he says, " with 

 the greater degree of certainty be assured of the degrees of 

 compressibility of these different airs, I divided the capacities 

 of two equal tubes into quarters of cubic inches, by pouring 

 severally those quantities of water into the tubes, and then 

 cutting notches with a file on the sides of the tubes at the seve- 

 ral surfaces of the water ; by which means I could see, by the 

 ascent of the compressed water in the tubes, that both the fac- 

 titious and common air were exactly alike compressible in all 

 degrees of compressure, from the beginning till they were 

 loaded with a weight equal to that of three atmospheres, which 

 was the furthest I durst venture for fear of bursting the glass*." 

 Having made this contribution to our knowledge of the phy- 

 sical properties of the gases, and established that at common 

 pressures and temperatures " with equal weights they are com- 

 pressed exactly in the same proportion with common air," he 

 went on to examine whether there exists any difference of spe- 

 cific gravity between the air and them ; but contenting him- 

 self with the single experiment to which I have already re- 

 ferred, where no difference could be detected t» he left to Caven- 

 dish the grand discovery of the distinctions of density in elastic 

 fluids; and it may possibly increase your respect for that dis- 

 covery to remark that his false conclusion led him into much 

 error in computing the weight of aerial substance fixed in va- 

 rious bodies from the volume which they yielded, on the sup- 

 position that the density of all airs is the same. 



* Stat. Essays, Append, p. 314. 

 f Analysis of the Air, Exp. 77. 



