94 Mr Dalton's Phys'wlogicdl Investigations on 



and that certain laws of equilibrium take place, by which water 

 acquires that state in which it is disposed neither to give out 

 nor to take in any more gas, have been abundantly proved by 

 Dr Henry and myself. M. Saussure has shown the like for 

 other liquids, and for a great number of solid bodies. It may 

 be seen, too, in my Chemistry, vol. i. p. 236, that a bladder, which 

 is generally considered as an animal membrane, least pervious 

 to air, may be filled with one gas, and being some time exposed 

 to the atmosphere, it will be found to continue full blown as at 

 first, but the contents will be chiefly atmospheric air. Messrs 

 Allen and Pepys, in their ingenious and excellent essays on respi- 

 ration, have proved that when a Guinea pig or a pigeon is con- 

 fined for an hour, more or less, in a mixture of hydrogen and 

 oxygen gases, in proportion as 78 to 22, a large portion of azotic 

 gas is found in the residue, and an equal portion of hydrogen 

 disappears. They ascribe this change to effects of respiration, 

 but it appears to me more probably due to the principle we are 

 advocating ; namely, to the egress of azotic from the whole body, 

 and the ingress of hydrogen in lieu of it, in consequence of with- 

 drawing the external pressure of the former and substituting 

 that of the latter. 



When the palm of the hand is placed over the top of the 

 receiver of an air-pump, and the air is exhausted, the pressure 

 of the air on the outside is scarcely felt, but the inside is swollen 

 and feels as if it was drawn or sucked into the receiver. Thus 

 the sensation is on the inside and not without ; but there is 

 within, and the consequence is a tendency of the air in the hand 

 to escape into the receiver, which occasions the pain and swell- 

 ing. It is thus also that the issuing of blood in the surgical 

 operation of cupping is effected. 



Though it does not seem of much consequence what the 

 pressure of the air may be on the animal frame within certain 

 limits, yet sudden changes must always be accompanied with 

 uneasy sensation. Climbing mountains, or ascending in a bal- 

 loon, removes a part of the atmospheric pressure from the body ; 

 this causes the air in the body to tend outwards, and sometimes oc- 

 casions bleedings. To supply oxygen to the lungs, a greater 

 volume of air must be breathed, and this seems to produce an 

 acceleration of the pulse. On the other hand, by descending 



