COMPOSITION OF THE ATMOSPHERE. 39 



Chemical Society* for 1858^ that Dr. Eoscoe found 1200 

 to be nearer the demand in a hospitaL Dr. Amott, a re- 

 vered authority on ventilation, demands 1200. I shall 

 return to this question. 



Carbonic Acid of the Air in England. 



It may be allowed first to quote a calculation of mine in 

 the 'Chem. Soc. Journal/ 1859. 



Allowing the air to go at the average rate of twelve miles 

 an hour*, it will sweep over the three or four miles of 

 Manchester three times an hour, or thirty-six times in 

 twelve hours. 



Taking the height of air aflfected to be 300 feet, as we 



must assume something, we have carbonic acid — 



From coals 0*0091 per cent. 



„ expired air 0*0002 „ 



Call the usual amount o"o6 ... o"o6 „ 



0*0693 ,„ 



But call the usual amount 0*03, as it sometimes is, or 

 even below it, we have — 



From coals o'oo9i 



„ breathing 0*0002 



„ usual air 0*0300 



0*0393 P^J" cent, or 0*393 per thousand. 



We see then tbat the combustion of coal and the breathing 

 wiU produce carbonic acid amounting only to a third of that 

 in the purest air generally found, or one-fourth of that in 

 air having 0*04. It is certainly remarkable that the amount 

 which I calculated as that which ought to exist should be 

 found, by experiment, to be correct to the third decimal 

 place, according to Dr. Roscoe. At the same time my own 

 numbers are a little higher, and in extreme cases much 

 higher. 



Dr. Uoscoe has found the carbonic acid to be as low as 

 0*027 per cent, outside of Manchester in wet weather, and 

 * Mr. Hartnup, F.K..A.S., finds it 12*62 at Liverpool. 



