28 



ties a boundary is set to our experience, which 

 succeeding- ages may extend, without ever being' 

 able to comprehend the whole. 



The process, which in respiration changes the 

 dark venous blood into crimson coloured blood, 

 was first examined by CIGNA, and not without 

 success ; and after our great SCHEELE had taught 

 us the composition of the atmospheric air, and de- 

 monstrated as well the necessity of one of its consti- 

 tuents for the support of life, as the insufficiency of 

 the other, the change of the air in the lungs was 

 explained by LAVOISIER, MENZIES, and GOOD- 

 WYN, in a very satisfactory manner. They found, 

 that oxygen gas was consumed, and that its place 

 was filled up by carbonic acid gas ; as also, that 

 the expired air contained a very considerable 

 quantity of aqueous vapour. From this circum- 

 stance, LAVOISIER concluded, that the dark ve- 

 nous blood contained a combination of carbon 

 and hydrogene, which imparted to it the dark 

 colour ; and which, when exposed to air, became 

 oxygenized, and formed carbonic acid and water f 

 whereby part of the water, which accompanies 

 the expired air, was produced ; while the other 

 part evaporated from the humid membrane of 



