COMPOSITION OF THE ATMOSPHERE. 49 



senses to observe. The senses observe a diflFerence between 

 Manchester and the outskirts. The difference is 0*0034 

 per cent. The senses observe it in London, where the dif- 

 ference between the streets and parks is 0*0040 per cent. 

 They observe it also on the Thames and in wet weather. 

 But they do not observe it in Munich, which has more 

 carbonic acid than even these towns, and more than the 

 New Cut or Lower Thames 'Street. The conchision is, 

 that carbonic acid in these small amounts is not that which 

 annoys us. In some towns it is no doubt sulphurous acid, 

 in others organic matter and gases from putrefaction. 



It does not follow that we must therefore neglect carbonic 

 aci^ ; on the contrary, it ought to be examined minutely, so 

 that not the smallest increase be allowed, if possible ; not 

 that we know certainly of any positive evil which it can 

 do of itself in these small quantities, but because it almost 

 always comes in bad company. 



In the above analyses the air containing '0774 is really 

 worse than that containing '1604 and even 0*3, because 

 over the middens there is a little sulphuretted hydrogen. 

 It is well, then, in such cases to use a double test. Indeed 

 it is probable enough that other gases besides sulphuretted 

 hydrogen, such as marsh gas and hydrogen, products of 

 decomposition, are issuing from cesspools and middens. 

 I should not say probable; it is really certain. These 

 gases, including the carbonic acid, show the reason why 

 less oxygen should be found in such places. 



I believe these analyses are of importance in the inquiry 

 into the state of the air of all places, as they teach us the 

 meaning of a deviation from the normal amount of carbonic 

 acid as well as of oxygen in the air. A deviation of 0*2 is 

 painful to us when it is caused by simple want of ventila- 

 tion. If it is accompanied with gases of putrefaction, it 

 is much more hurtful, as some of these are very deadly. 



These analyses teach us to be very careful not to allow 



SER. III. VOL. III. E 



