DISSOLVED MATTER COXTAIXED IN RAIX-WATER. 141 



it passed into the condition of vapour, be comparatively pui'e, 

 altliough it is very doubtful whether under any condition it would 

 be possible, even if it suffered no further contamination to obtain 

 vapour water perfectly pure from rain. The surface from which the 

 water is formed, whether land or water, always contains more or less 

 combined nitrogen, which when the water evaporates, passes away 

 with it. The troublesome operation of preparing distilled water, 

 that will have no etlect on a sensitised Nessler solution, is one 

 well known to all conversant with water analysis. The opinion 

 generally accepted is that water vapour is made up of minute 

 spheres of liquid and that it is the coalescence of these spheres 

 which forms rain. We know that in the process of evaporation 

 that a considerable portion of the dissolved substances in the 

 licjuid are left behind, but in the case of gases thei'e seems to be a 

 limit, especially in the case of those like ammonia and carbonic 

 anhydride. 



The greater part of the dissolved matters in rain is derived 

 from the air through which the rain passes, and these may have 

 quite a different source from that of the water vapour itself. 

 This view is supported by the fact that the first rain falling after 

 a dry period is always found to be richer in dissolved matters 

 than that which falls subsequently, the proportion present 

 gradually diminishing as the shower proceeds. The rain thus 

 washes, as it were, the air, and any interval between two rainfalls 

 allows the air, moi'e or less according to the time, to become 

 again contaminated, so that several falls of short duration, might 

 contain, comparatively, more impurities than when the rain is 

 continuous for a longer period. This is well shown in our results 

 by the different quantities of rain falling in the several, months. 



The combined nitrogen contained in rain is derived from three 

 sources ; the ammonia compounds derived from tho decay of 

 animal and vegetable substances and from the combustion of fuel ; 

 the organic matter existing in the air ; and lastly the nitric acid 

 resulting either from the oxidation of ammonia, and probably 

 some of the organic matter, or, from the direct union of atmos- 

 pheric oxygen and nitrogen under the influence of the electrical 

 discharges taking place in the atmosphere. 



In the decay of animal and vegetable substances, and also in 

 combustion, the nitrogen present is evolved in the form of 

 ammonia, and the carbon as carbonic anhydride, these two 

 gases pass into the atmosphere and unite, forming carbonate of 

 ammonium, which being volatile at the ordinaiy temperature of 

 the air, is held as it were in suspension, until removed by the 

 rain. 



The organised nitrogen exists in the air in the form of germs 

 and minute organisms and possibly of minute particles of disin- 

 tegrated organic matter. Being generally in the form of 

 albuminoids the tei'm albuminoid nitrogen has been applied to it. 



