from Water exposed to Light. 195 



The method I have here adopted of using algebraic 

 characters in noting the result of the experiments made 

 to determine the goodness of air, though not strictly 

 mathematical, is very convenient ; and for that reason I 

 shall continue to make use of it. a represents the air 

 which is proved; n nitrous air; and the numbers which 

 are joined to these letters shew the quantities or the 

 number of measures of the different airs made use of in 

 the experiment. The other number, which stands alone, 

 or without any letter attached to it, on the other side of 

 the equation, shews the volume, or the number of meas- 

 ures and parts of a measure to which the two airs are 

 reduced after they are mixed. I shall sometimes add a 

 fourth number, shewing the quantities of the two airs 

 destroyed, as this more immediately shews the goodness 

 of the air which is proved. 



Thus, in the experiment last mentioned, r measure of 

 the air proved, mixed with 4 measures of nitrous air, 

 were reduced to 1.35 measure; consequently, 3.65 meas- 

 ures of the two airs were destroyed, for it is i -(-4 = 5 

 1.35 = 3.65; and the result of this trial I should 

 write thus, I a -(- 4 n = 1.35 or 3.65. 



Or, for still greater convenience in practice, as this last 

 number 3.65, or 3 T Vo 5 shews more immediately the 

 goodness of the air in question, as I have just observed, 

 by supposing with Dr. Ingen-Housz the measure of the 

 eudiometer to be divided into 100 equal parts, it will 

 be 100 a -{- 400 = 135 ; and 365, expressing the 

 volume of the two airs destroyed, will become a whole 

 number. 



But, instead of writing i oo a -|- 400 = 135, &c., I 

 shall continue to write i ^ -j- 4 = 1.35, and shall ex- 

 press the last number (3.65) as a whole number notwith- 



