308 OF THE CIRCULATORY SYSTEM. SECT. XXIII. 4. 



rhceas, as difagreeing food is vomited from the do- 

 mach. 



IV. As all the fluids, that pafs through thefe 

 glands, and capillary veflels, undergo a chemical 

 change, acquiring new combinations, the matter of 

 heat is at the fame time given out ; this is apparent, 

 fmce whatever increafes infenfible perfpiration, in- 

 creafes the heat of the ikin ; and when the adion of 

 thefe veflels is much increased but for a moment, as 

 in blufhing, a vivid heat on the (kin is the immediate 

 confequence. So when great bilious fecretions, or 

 thofe of any other gland, are produced, heat is 

 generated in the part in proportion to the quantity 

 of the fecretion. 



The heat preduced on the fkin by blufhing may 

 be thought by fome too fudden to be pronounced a 

 chemical effect, as the fermentations or new com- 

 binations taking place in a fluid is in general a 

 flower procefs. Yet are there many chemical mix 

 tures in which heat is given out as inftantaneoufly ; 

 as in folutions of metals in acids, or in mixtures of 

 eflential oils and acids, as of oil of cloves and acid 

 of nitre. So the bruifed parts of an unripe apple 

 become almofl inflantaneoufly fweet ; and if the 

 chemico-animal procefs of digeftion be flopped for 

 but a moment, as by fear, or even by voluntary 

 erudation, a great quantity of air is generated, by the 

 fermentation which inflantly fucceeds the flop of 

 digeftion. By the experiments of Dr. Hales it appears, 

 that an apple during fermentation gave up above fix 

 hundred times its bulk of air ; and the materials in 

 the flomach are fuch, and infuch actuation, as im- 

 mediately to run into fermentation, when digeflion 

 is impeded. 



As the blood paffes through the fmall veflels of the 

 lungs, which connect the pulmonary artery and 

 vein, it undergoes a change of colour from a dark to 



a light 



