39 
Ammonia was discovered in 1774 loy Priestley, but the 
composition was ascertained by Berth ollet in 1775, by 
splitting it into its constituent elements by means of elec- 
tricity, when he came to the conclusion that it contained 
0’19o parts by weight of hydrogen to 0‘807 parts by weight 
of nitrogen. Dalton assumed that this substance is a com- 
pound of one atom of hydrogen with one of nitrogen, and 
807 X 1 
hence he obtained for the atomic weight of azote — = 4’2 ; 
and 4‘2-l~l = 5‘2 as the atomic weight of ammonia. It is 
also probable that Dalton made use of the composition of 
the oxides of nitrogen for the purpose of obtaining the 
atomic weight of nitrogen. If we take the numbers ob- 
tained partly by Davy and partly by himself, as given on 
page 318 of the New System, as representing the composition 
of the three lowest oxides, it appears that the mean 
value for nitrogen is 4’3 when oxygen is taken as 5*5. In 
all probability the number in this table (4*2) was obtained 
from an experiment of Dalton’s made at an earlier date. 
It is not possible to ascertain the exact grounds upon 
which Dalton gave the number 7 *2 for phosphorus; its 
juxtaposition, however, in the table to phosphuretted hydro- 
gen shows that it was probably an analysis or a density 
determination of this gas which led him to the atomic 
weight 7'2, under the supposition that this gas (like am- 
monia) consisted of one atom of each of its components. In 
the second table, published in 1808, Dalton gives the number 
9 as that of the relative weight of the phosphorus atom, 
and we are able to trace the origin of this latter number, 
although that of 7*2 is lost to us. On p. 460, Part II. of 
his New System, Dalton states that he found 100 cubic 
indies of phosphuretted hydrogen to weigh 26 grains, the 
same bulk of hydrogen weighing 2*5 grains ; hence, assuming 
that equal volumes contain an equal number of atoms, we 
have : 
26—2*5 
:9'4 gives the atomic weight of phosphorus' 
2*5 
