COMPOSITION OP THE ATMOSPHERE. 51 



the highest on the plains, less at looo and 2000 feet, and 

 more again at 3000 to 4000 feet, making differences of three 

 in a million, and so at 3000 feet beginning to increase, as 

 other observers have stated to be the case at great eleva- 

 tions. 



Mode of Reading the Numbers. 



When vre read of a fraction per cent., we imagine the 

 amount to be very small ; and if the fraction is in the second 

 decimal place, we have no faith in it. We are quite wrong. 

 The amount of sulphuric acid in the air runs into the fourth 

 or fifth decimal place, and the actual sulphur in it still 

 further removed, and yet a greater effect is produced on 

 the atmosphere than is indicated by the amount shown in 

 the oxygen or carbonic acid volumes. In speaking of these 

 gases, we have been accustomed to disregard minute differ- 

 ences, and one- tenth of a per cent, of oxygen more or less 

 is indifferently given or taken. It may, however, assist us 

 if we disregard the decimal point, and in the oxygen volume 

 with three decimal places read the number as full and parts 

 in 10,000 : we shall then find that the difference between 

 21 "00 per cent, of oxygen and 20*90 is really 10 in 10,000 

 of the air and 0*47 per cent, of the oxygen. This amount 

 in the case of some gases -is simply intolerable ; and although 

 not perceptible as regards oxygen, it may not be less effi- 

 cient. I conclude from these inquiries that we must pay 

 attention to the number in the second place for oxygen : 

 perhaps at a future time we may arrive at the third. 



In the case of carbonic acid we must attend to the third 

 even now, as I believe, and even to the fourth or one part in 

 million. The numbers are all put down as fractions, and 

 may be read as fractions per cent. ; but they may all be 

 read as whole numbers, disregarding the decimal point. 

 Thus "0314 will mean 314 in a million. Between this and 

 •0400 we have 86 in a million, which is no trifling amount 



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