THE AIR OF TOWNS. 355 



a minute quantity of impurity may produce effects apparently quite dis- 

 proportionate to the cause. We have it on the authority of Professor 

 Koberts-Austen that a difference of one-tenth per cent, of carbon in 

 steel rails may be a very serious matter. 



The steel cylinder, containing compressed oxygen, which recently 

 burst at the station at Bradford with such fatal effect, contained only 

 three-tenths per cent too much carbon — an amount, however, quite suf- 

 ficient to account for the mischief. 



The steel dies used in the mint should strike 40,000 coins on the aver- 

 age, yet if the die contained one-tenth too much carbon it would not 

 strike 1 00 pieces without cracking. 



Let us see what is the full effect of the difference in carbonic acid in 

 town and country air. If we take country air to contain 0.03 and town 

 air 0.04 per cent of carbonic acid, or a difference of 0.01 per cent, it will 

 amount to about 1 additional quart of carbonic acid inhaled during the 

 day, supposing we take into our lungs 2,600 gallons of air per diem. 



This would weigh about 30 grains, an amount sufficient to kill ten 

 people if the poison were as virulent as white arsenic. Moreover, we 

 must remember that if we inhale 1 quart of carbonic acid more we take 

 in 1 quart less of life-supporting oxygen. Is carbonic acid really so pois- 

 onous that a quart or gallon more carbonic acid and a corresponding 

 amount of oxygen less would be hurtful to this extent! The answer 

 is a Ko." Although from experiments made by Angus Smith in an air- 

 tight leaden chamber, when pure carbonic acid was introduced to the 

 extent of 3.84 per cent, two friends suffered after a few minutes from 

 headache, and he himself soon felt great discomfort, it is known that 

 workers in soda-water factories, where the amount of carbonic acid in 

 the air reaches 0.1 per cent, are not injuriously affected. Yet our senses 

 detect the difference between town and country air. We can perceive 

 the difference between Manchester town air and that of the outskirts — 

 a difference of only 0.0034 per cent — or between the air of the streets 

 and the parks of London, which amounts to 0.004 per cent. Why can 

 we detect these minute differences? Because, as Angus Smith says, 

 carbonic acid always comes in bad company. It is its bad companions 

 that affect us. It is the sulphurous acid, which accompanies burning 

 coal and gas; it is the organic poison which accompanies the exhala- 

 tions from the body. 



The latter is the subject to which I now wish to direct your attention. 



It is obviously very important to determine minute differences of car- 

 bonic acid in the air so that we may guard against the least increase 

 in carbonic acid in the atmosphere. As little as 0.004 per cent can be 

 detected by our senses, as we have seen, and a difference of 0.02 per cent 

 is not pleasant when caused by want of ventilation. Angus Smith says : 

 "We all avoid an atmosphere of 0.1 per cent in a crowded room, and the 

 experience of civilized men is that it is not only odious, but unwhole- 

 some. W T hen people speak of good ventilation in dwelling houses they 



