ScTEXTiFic Lectures. 253 



the Inngs to receive another portion of oxygen. Thus, time after 

 time, tliese indefatigable httle hiborers bear their bnrden of life, giving 

 gas to the tissue of the body, until at last they are woiti out in the 

 service, and dying in the performance of tlicir duty, finally pass 

 away to give room to new generations of discs, that have arisen and 

 usurped their places and duties. 



Though oxygen is thus continually inti'odnced into the system in 

 considerable quantities, carefully conducted experiments have shown 

 that only small traces of nitogen are a1)Sorbed and exhaled, and these 

 are so insignificant that tliey are generally ignored. We may, there- 

 fore, say that the cliief use of the nitrogen is to dilute the oxv<i;en, 

 and as we reduce the strength of a tiery alcholic liquid by the addi- 

 tion of water, so the combustion supporting power of oxygen, is 

 reduced by mingling it with diluting nitrogen and an air of mild and 

 suitable properties formed. 



From the invariable we pass to tlie varialde components ; though 

 these are small in ipiantity, they are very important, carlionic acid 

 and carbonic oxyd occupying a prominent position. To the first 

 of these your attention is now directed. 



Again we find that the subject of our examination is a colorless 

 gas, as is evidenced by the jar of carbonic acid now before you. It 

 looks like either oxygen or nitrogen, but if we taste and smell it, we 

 find that it affects both of these senses, possessing a pungent agree- 

 able taste, and an odor of a like nature. Testing it by the flame, 

 the latter is extinguished, tlie gas acting in this respect like nitrogen, 

 but differing from it in that, it is so heavy that it nniy be poured 

 from one vessel into another, where its presence may be shown by a 

 Hame. Another test for the presence of carbonic acid is lime water, 

 to which it im])arts a milky appearance, as is demonstrated by pass- 

 ing some of tlic gas through the colorless lime water contained in 

 til is vase. 



If we investigate the action of carbonic acid on living creaturea 

 we find that if it is concentrated it quickly destroys the life of a 

 warm blooded animal, and even when very dilute, acts as a narcotic 

 poison. ITnder ordinary circumstances it occurs to the extent of from 

 two to five parts in 10,000 of air, but in theatres and ]>ublic halls the 

 proportion sometimes rises to 100 parts in ten thousaiid. Of its copi- 

 ous production and accumulation, in crowd(;d rooms, we mav r(>adilv 

 satisfy ourselves by passing the air of such an apartment through a 

 column of colorless lime \vater, a,s in the experiment we have here 

 arranged, when at once its presence is made evident. 



