E. H. Richards 337 



with air at the lower temperature. With a small undisturbed surface 

 relative to volume of water aeration is a very slow process. The obser- 

 vations made by Passerini^ on the difference of temperature of air and 

 rain at the moment of falhng, show that in Italy rain is normally about 

 2° cooler than the air, but occasionally the diiTerence is as much as 10° C. 



It seems to be clear that summer rain, unhke that which falls during 

 the rest of the year, is not fully saturated with oxygen. It is difficult 

 to understand the reason for this. The relative temperatures of the 

 rain clouds and of the air at the ground level in summer and winter, if 

 they have any influence at all, should produce a result exactly opposite 

 to that actually found, i.e. summer rain ought to be supersaturated 

 when collected. 



We may conclude that rain-water is very nearly saturated with 

 oxygen when its temperature as collected is below 15° C. as is the case 

 for about nine months in the year in this chmate. When the tempera- 

 ture of the rain is above 15° C, the dissolved oxygen is always below 

 saturation, occasionally as much as 25 per cent. 



The author wishes to acknowledge the help given him with the 

 literature of the subject by the late Dr N. H. J. Miller. 



^ Bolletino della Scuola Agraria di Scandicci, vol. i, 1893, p. 95; also vol. ii, p. 65. 



{Received Janvxiry 30lh, 1917.) 



