[shutt-dorrance] nitrogen COMPOUNDS IN RAIN AND SNOW 65 



when the wind is from the north, and there is no doubt that the pre- 

 cipitation occurring while these fumes are in the air must be con- 

 taminated to some degree. 



The catchment area for the rain, consisting of a slightly concave 

 lead-lined tray 60" x 30", is on the north side of the laboratory, outside 

 a second-story window. The rain is conducted through glass tubing 

 to a large bottle inside the building and an analysis is made of every 

 precipitation yielding sufficient water (prec. 0.01" or more) for 

 analysis. This analysis is made as soon as possible after collection. 

 In cases of continued precipitation collections are made at 8 A.M. 

 and 5 P.M. The snow is collected on the granolithic walk leading 

 to the laboratory and as the walk is shovelled clean after each fall 

 the sample is not contaminated by snow which has been exposed for 

 some time. The snow is put in a tall glass jar with a closely fitting 

 cover, and allowed to melt in the laboratory. 



It would appear to have been the practice with many investigators 

 to make one analysis of a composite sample representing the precipita- 

 tion for a month, using a preservative such as formaldehyde or mercu- 

 ric bichloride. This method, however, while minimizing the analyti- 

 cal work, has in our ejcperience objections, the results being markedly 

 at variance with the totals obtained from the immediate analysis of 

 each precipation — the plan followed in our work. This indicates that a 

 change takes place in the collected rain in spite of the presence of a 

 preservative. 



No trouble with birds roosting' on the catchment area has been 

 experienced, due perhaps to the close proximity of the farm barns, 

 orchards and shade trees. We have thus been free from a difficulty 

 interfering with the accuracy of the results met with by other workers 

 in this enquiry. 



During the decade 897 samples were analysed; of these 616 were 

 of rain, and 281 of snow. 



Factors influencing the Composition of the Precipitation. 



In many cases a black deposit has been observed in the con- 

 tainer on allowing the rain water to stand — apparently similar in 

 character to that which has been remarked in the case of the snow 

 collecting. As the tray is frequently scrubbed this cannot be due to 

 accumulated dirt, but must consist of material washed out of the air. 

 We conclude that both in summer and winter the atmosphere is more 

 or less charged with solid particles, soot, etc., of which it is purified, 

 in part, at least, by the precipitation. 



Sec. Ill, Sig. 5 



