PREFACE. V 



Finally, in Chapter VI, I return to the problem begun in Chapter I, 

 to see whether there is any change in the colloidal nucleation of dust- 

 free air, such as might be ascribed to the ionization produced by some 

 penetrating cosmical radiations coming from without; for it was to 

 be the plan of these researches to study the ordinary dust content of 

 the atmosphere with regard to its variation in the lapse of time first, and 

 thereafter to continue in the same way with the nucleation of the dust- 

 free atmosphere. This nucleation is found to increase synchronously 

 with the decrement of the barometer; but as the amount of adiabatic 

 cooling, i. e., the efficiency of the apparatus {cat. par.), follows the same 

 conditions, it is extremely difficult to disentangle the two effects. Never- 

 theless the results are of considerable interest and they are therefore 

 reported in their present state of progress. 



My thanks are due to Miss L. B. Joslin, who not only gave efficient 

 assistance in the preparation of the manuscript and of the drawings for 

 the press, but contributed much of the work in Chapter III, section 62, 

 and Chapter V. 



Carl Barus. 



