Original Letters of Dr. Franklin. 165 



sides, water agitated ever so violently produces no heat, as 

 has been found by accurate experiments. 



3. A hollow sphere of lead, has a firmness and consisten- 

 cy in it, that a hollow sphere of fluid unfrozen water cannot 

 be supposed to have. The lead may support the pressure 

 of the water 'tis immerged in, but the bubble could not sup- 

 port the pressure of the air if empty within. 



4. Was ever a visible bubble seen to rise in air? I have 

 made many when a boy with soap suds, and a tobacco pipe ; 

 but they all descended when loose from the pipe, though 

 slowly, the air impeding their motion. They may indeed 

 be forced up by a wind from below, but do not rise of them- 

 selves though filled with warm breath. 



5. The objection relating to our breathing moist air, 

 seems weighty, and must be farther considered. The air 

 that has been breathed has doubtless acquired an addition 

 of the perspirable matter, which nature intends to free the 

 body from, and which would be pernicious if retained, or 

 returned into the blood. Such air then may become unfit 

 for respiration, as well for that reason, as on account of its 

 moisture. Yet I should be glad to learn by some accurate 

 experiment, whether a draft of air two or three times in- 

 spired and expired, (perhaps in a bladder) has, or has not 

 acquired more moisture than our common air in the damp- 

 est weather. 



As to the precipitation of water in the air we breathe, 

 perhaps it is not always a mark of that air's being overload- 

 ed. In the region of the clouds, indeed, the air must be 

 overloaded (its coldness considered) if it lets fall its water 

 in drops, which we call rain; but those drops may fall 

 through a dryer air near the earth; and accordingly we 

 find, that the hygroscope sometimes shows a less degree of 

 moisture during a shower, than at other times when it does 

 not rain at all. The dewy dampness that settles on the 

 insides of our walls and on our wainscots, seems more cer- 

 tainly to denote an air overloaded with moisture, and yet 

 this is no sure sign. For after a long continued cold sea- 

 son, if the air grow suddenly warm, the walls, &c. continu- 

 ing their coldness longer, will for some time condense the 

 moisture of such air, 'till they grow equally warm ; and 

 then they condense no more, although the air is not become 

 dryer. And on the other hand, after a warm spell, if the 

 air grow cold, though moister than before, the dew is not 



