54 HOW CROPS FEED. # 
(5) by giving rise to the furmation of white fumes, when any object (e. ¢., 
a glass rod) moistened with hydrochloric acid, is brought in contact with 
it. These fumes arise from the formation of solid ammoniacal salts pro- 
duced by the contact of the gases. 
b. Nessler’s Test.—For the detection of exceedingly minute traces of 
ammonia, a reaction first pointed out by Nessler may be employed. Di- 
gest at a gentle heat 2 grammes of iodide of potassium, and 8 grammes 
of iodide of mercury, in 5 cub. cent. of water; add 20 cub. cent. of wa- 
ter, let the mixture stand for some time, then filter; add to the filtrate 
30 cub. cent. of pure concentrated solution of potassa (1 : 4); and, should 
a precipitate form, filter again. If to this solution is added, in small 
quantity, a liquid containing ammonia or an ammonia-salt, a reddish brown 
precipitate, or with exceedingly small quantities of ammonia, a yellow 
coloration is produced from the formation of dimercurammonic iodide, 
NHe, I.OHs. 
ec. Bohlig’s Test.—According to Bohlig, chloride of mereury (corrosive 
sublimate) is the most sensitive reagent for ammonia, when in the free 
state or as carbonate. It gives a white precipitate, or in very dilute so- 
lutions (even when containing but #|200,o00 of ammonia) a white turbidity, 
due to the separation of mercurammonic chloride, NH, Hg.Cl. In solu- 
tions of the salts of ammonia with other acids than carbonic, a clear 
solution of mixed carbonate of potassa and chloride of mereury must be 
employed, which is prepared by adding 10 drops of a solution of the 
purest carbonate of potassa, (1 of salt to 50 of water,) and 5 drops of a 
sulution of chloride of mereury to 80 c¢. ec. of water exempt from am- 
monia (such is the water of many springs, but ordinary distilled water 
rarely). This reagent may be kept in closed vessels for a time without 
change. If much more concentrated, oxide of mercury separates from it. 
By its use the ammonia salt is first converted into carbonate by double 
decomposition with the carbonate of potassa, and the further reaction 
proceeds as before mentioned. 
Occurrence of Ammonia in the Atmosphere,—The ex- 
istence of ammonia in the atmosphere was first noticed by 
De Saussure, and has been proved repeatedly by direct 
experiment. That the quantity is exceedingly minute has 
been equally well established. 
Owing partly to the variable extent to which ammonia 
occurs in the atmosphere, but chiefly to the difficulty of 
collecting and estimating such small amounts, the state- 
ments of those who have experimented upon this subject 
are devoid of agreement. 
We present here a tabulated view of the most trust- 
worthy results hitherto published : 
