28O REDUCTION TO AIR. 



For a quotient, the minute correction is obtained by 

 subtracting the correction of the divisor from that of the 

 dividend, as is well known. 



Let us apply this process to the case of arsenic. 



The dividend is 234, the absolute weight of the salt, 

 formed from 354 of pyroarsenate, the divisor. 



The specific gravity of salt is 2.16, which gives its factor 

 0.41 milligrammes per gramme, allowing for brass weights. 



The specific gravity of the pyroarsenate was (by Hibbs) 

 taken at 2.295 ^ or which we take 2.30; this gives the factor 

 0.38 milligrammes per gramme. 



Reducing to air, makes the sign opposite; hence result- 

 ing factor for the fraction is 



0.41 ( 0.38) which is 0.03, 



that is a subtraction of 0.03 milligrammes per gramme, or 

 3 units in the fifth place. 



But the ratio is 0.66 102 for which we take here 0.66, 

 that is %. 



Now % times 3 is 2 ; the sign being negative, we see that 

 in this case 



the reduction of our absolute atomic ratio to air is 

 obtained by adding tivo units in the fifth decimal. 



If now, we wish to reduce the actual weighings given 

 p. 207 to vacuum, we simply need to subtract two units in 

 the fifth place. 



The table, p. 230, will now show how remarkable are the 

 errors of reduction actually committed. 



Of course, we must retain our absolute atomic ratios 

 pure and simple, and merely calculate this reduction to air 

 in the manner shown to be able to allow for actual 

 differences observed. 



As the slight variations produced by changing tempera- 

 ture and pressure fix only the correction of a correction, 

 that is, a minute quantity of second order, they are usually 

 entirely insignificant. Sufficient data for the allowance are 

 given in my General Chemistry, 231, note. 



To complete this subject, we state how the minute factor 

 referred to above is actually obtained. 



