122 Rev. W. V. Harcourt on Lord Brougham's statements 



was, that the elasticity of the air had been destroyed. Dr. 

 Hales, dissatisfied with so loose an explanation, accounted for 

 the loss, which in the case of nitrous acid he first observed, 

 after the following manner : — " When fresh air is let into the 

 receiver, whose included air is impregnated with the fumes 

 arising from the mixture of compound aquafortis, or spirit of 

 nitre, and Whitstable pyrites, mentioned in the following ex- 

 periment, then the air in the receiver turns very red and tur- 

 bid, and much air is absorbed after several repeated admis- 

 sions. When fresh air is thus admitted into the glasses full 

 of sulphureous, though clear, air, a good many particles of the 

 fresh air must needs be reduced by the sulphureous ones from 

 an elastic to a fixed state, as in the effervescences of other 

 liquors. Therefore the rising of the water in the glass vessel 

 does not seem to be wholly owing to the rebating of the air's 

 elasticity in some degree, but rather to the reduction of it from 

 an elastic to a fixed state, which is further probable from 

 hence, viz. that the whole quantity of air admitted at several 

 times is equal, or nearly equal, to the quantity of sulphureous 

 air A. Z., so that both airs are at the same time contained 

 within the space A. Z." 



In this important observation, which was subsequently 

 turned to such good account by Priestley and Cavendish, 

 Hales gave the true theory of the loss of volume which oc- 

 curs by the admission of common air to nitrous gas; but the 

 variable, and apparently capricious, loss of elasticity which he 

 remarked in other gases he could not explain. " Though a 

 good part of the air," he says, " which rises from Jluids seems 

 to have existed in an elastic state in those fluids, yet the air 

 which arises from solid bodies, either by the force of fire or 

 effervescence, does not seem to arise only from the interstices 

 of those bodies, but principally from the most fixed parts of 

 them. For since the airs which are raised by the same acid 

 spirit from a vast variety of substances have very different 

 degrees of permanency, as was shown in Exp. 10, No. 3, 4, 

 5, 6, and in Exp. 11, No. 6, 7, 8, 9, 10 of experiments on 

 stones, hence it is probable that these airs do not arise from 

 latent interstices of the dissolved stones, &c, but from the solid 

 fixed particles of them; and since the whole of some of these 

 newly-generated airs does in a few days lose its elasticity, it 

 should seem hence probable, that whatever air arises from the 

 spirit in the effervescence is not permanently elastic, or else 

 that in the rotation of some stones it is thrown off into a more 

 permanently elastic state than from others." 



The cause of this loss of volume was first explained in Ca- 

 vendish's paper : he proved by experiment, that carbonic acid 



