236 PHILOSOPHICAL TEANSACTIONS. [aNXO 1777. 



mean rate of expansion of air of that extraordinary density is 2.0471 7 for each 

 degree. 



From the 1st set of the 2d class of experiments it appears, that lUOO equal 

 parts of air, pressed only with f of an atmosphere, and suffering a heat of 212°, 

 expands nearly 484 of those parts, of which the mean rate for each degree is 

 2.28140. The maximum corresponds to that section of the scale between 52° 

 and 72°; and the rate for the extremes is less than the mean. 



But in the 2d set of this class, when the confined air was rendered so ex- 

 tremely rare as to be pressed with only 4- of an atmosphere, in which case there 

 was a necessity for heating it red-hot before it was possible to make the quicksilver 

 hang in any tube of a moderate length, the expansion of 1000 equal parts of 

 air was, by the 7th and 8th experiments, diminished to about f of the usual 

 quantity; and by the Qth, it was considerably less, amounting only to 141.5 for 

 the 180° comprehended between freezing and boiling, or 0.78613 for each de- 

 gree. The maximum still corresponds to the space between 52° and 72°; and 

 the minimum is constantly at the boiling point. 



From these last 3 experiments it would seem, that the particles of air maybe 

 so far removed from each other, by the diminution of pressure, as to lose a very 

 great part of their elastic force; since, in the 19th experiment, the heat of boil- 

 ing water applied for an hour together, could only make it occupy a space which, 

 compared with what it filled at freezing, bears the proportion of 1 141.5 to 1000. 



From the 3d class of experiments it appears, that common air, pressed with 

 a single atmosphere, whether taken into the manometer in its natural state, or 

 heated red-hot in it, has the same expansion with air of only a of that density: 

 for 1000 equal parts of this air expanded 484.21 from 212" of heat, of which 

 the mean rate is 2.28401 for each degree. By comparing this result with that 

 of the 1st class, and again with that deduced from the 2d set of the 2d class, 

 it would seem that the elastic force of common air, is greater than when its den- 

 sity is considerably augmented or diminishetl by an addition to, or subtraction 

 from, the weight with which it is loaded; for, in the first case, it bears the pro- 

 portion of 484 to 434; and in the last it is, from the mean of 3 experiments, 

 as 484 to 252, when pressed with only i of an atmosphere. 



As barometrical observations will probably never be made ui a temperature 

 higher than 92° in the shade, nor in one lower than 12°, if we subtract 26.038, 

 the expansion answering to 12°, from 222.006, that which corresponds to 92°, 

 we have 195.968 for the 80 intermediate degrees; or 2.45 for the mean rate on 

 each. This equation, compared with Mr. de Luc's, bears the proportion of 

 245 to 210, which is a difference of -r^-f-rTr on every degree, or a 7 th part of 

 the whole: and though this rate will be found hereafter to exceed that deduced 

 from the operations of the barometer in extreme temperatures ; yet they agree 



